BACKGROUND:Surgery for rotator cuff disease is usually used after non-operative interventions have failed, although our Cochrane Review, first published in 2007, found that there was uncertain clinical benefit following subacromial decompression surgery. OBJECTIVES:To synthesise the available evidence of the benefits and harms of subacromial decompression surgery compared with placebo, no intervention or non-surgical interventions in people with rotator cuff disease (excluding full thickness rotator cuff tears). SEARCH METHODS:We searched CENTRAL, MEDLINE, Embase, Clinicaltrials.gov and WHO ICRTP registry from 2006 until 22 October 2018, unrestricted by language. SELECTION CRITERIA:We included randomised and quasi-randomised controlled trials (RCTs) of adults with rotator cuff disease (excluding full-thickness tears), that compared subacromial decompression surgery with placebo, no treatment, or any other non-surgical interventions. As it is least prone to bias, subacromial decompression compared with placebo was the primary comparison. Other comparisons were subacromial decompression versus exercises or non-operative treatment. Major outcomes were mean pain scores, shoulder function, quality of life, participant global assessment of success, adverse events and serious adverse events. The primary endpoint for this review was one year. For serious adverse events, we also included data from prospective cohort studies designed to record harms that evaluated subacromial decompression surgery or shoulder arthroscopy. DATA COLLECTION AND ANALYSIS:We used standard methodologic procedures expected by Cochrane. MAIN RESULTS:We included eight trials, with a total of 1062 randomised participants with rotator cuff disease, all with subacromial impingement. Two trials (506 participants) compared arthroscopic subacromial decompression with arthroscopy only (placebo surgery), with all groups receiving postoperative exercises. These trials included a third treatment group: no treatment (active monitoring) in one and exercises in the other. Six trials (556 participants) compared arthroscopic subacromial decompression followed by exercises with exercises alone. Two of these trials included a third arm: sham laser in one and open subacromial decompression in the other.Trial size varied from 42 to 313 participants. Participant mean age ranged between 42 and 65 years. Only two trials reported mean symptom duration (18 to 22 months in one trial and 30 to 31 months in the other), two did not report duration and four reported it categorically.Both placebo-controlled trials were at low risk of bias for the comparison of surgery versus placebo surgery. The other trials were at high risk of bias for several criteria, most notably at risk of performance or detection bias due to lack of participant and personnel blinding. We have restricted the reporting of results of benefits in the Abstract to the placebo-controlled trials.Compared with placebo, high-certainty evidence indicates that subacromial decompression provides no improvement in pain, shoulder function, or health-related quality of life up to one year, and probably no improvement in global success (moderate-certainty evidence, downgraded due to imprecision).At one year, mean pain (on a scale zero to 10, higher scores indicate more pain), was 2.9 points after placebo surgery and 0.26 better (0.84 better to 0.33 worse), after subacromial decompression (284 participants), an absolute difference of 3% (8% better to 3% worse), and relative difference of 4% (12% better to 5% worse). At one year, mean function (on a scale 0 to 100, higher score indicating better outcome), was 69 points after placebo surgery and 2.8 better (1.4 worse to 6.9 better), after surgery (274 participants), an absolute difference of 3% (7% better to 1% worse), and relative difference of 9% (22% better to 4% worse). Global success rate was 97/148 (or 655 per 1000), after placebo and 101/142 (or 708 per 1000) after surgery corresponding to RR 1.08 (95% CI 0.93 to 1.27). Health-related quality of life was 0.73 units (European Quality of Life EQ-5D, -0.59 to 1, higher score indicating better quality of life), after placebo and 0.03 units worse (0.011 units worse to 0.06 units better), after subacromial decompression (285 participants), an absolute difference of 1.3% (5% worse to 2.5% better), and relative difference of 4% (15% worse to 7% better).Adverse events including frozen shoulder or transient minor complications of surgery were reported in approximately 3% of participants across treatment groups in two randomised controlled trials, but due to low event rates we are uncertain if the risks differ between groups: 5/165 (37 per 1000) reported adverse events with subacromial decompression and 9/241 (34 per 1000) with placebo or non-operative treatment, RR 0.91 (95% CI 0.31 to 2.65) (moderate-certainty evidence, downgraded due to imprecision). The trials did not report serious adverse events.Based upon moderate-certainty evidence from two observational trials from the same prospective surgery registry, which also included other shoulder arthroscopic procedures (downgraded for indirectness), the incidence proportion of serious adverse events within 30 days following surgery was 0.5% (0.4% to 0.7%; data collected 2006 to 2011), or 0.6% (0.5 % to 0.7%; data collected 2011 to 2013). Serious adverse events such as deep infection, pulmonary embolism, nerve injury, and death have been observed in participants following shoulder surgery. AUTHORS' CONCLUSIONS:The data in this review do not support the use of subacromial decompression in the treatment of rotator cuff disease manifest as painful shoulder impingement. High-certainty evidence shows that subacromial decompression does not provide clinically important benefits over placebo in pain, function or health-related quality of life. Including results from open-label trials (with high risk of bias) did not change the estimates considerably. Due to imprecision, we downgraded the certainty of the evidence to moderate for global assessment of treatment success; there was probably no clinically important benefit in this outcome either compared with placebo, exercises or non-operative treatment.Adverse event rates were low, 3% or less across treatment groups in the trials, which is consistent with adverse event rates reported in the two observational studies. Although precise estimates are unknown, the risk of serious adverse events is likely less than 1%.