OBJECTIVE: To investigate whether short message service (SMS) reminders reduce nonattendance in physical therapy outpatient clinics. DESIGN: Prospective single-blinded randomized controlled trial. SETTING: Two physical therapy outpatient departments in metropolitan acute public hospitals. PARTICIPANTS: Participants with an appointment in a physical therapy outpatient clinic and who provided a contact mobile telephone number were included. Participants were excluded if their appointment was scheduled for the same day on which they made the appointment. INTERVENTION: Participants allocated to the intervention group received an SMS reminder before their next appointment; participants allocated to the control group did not receive a reminder. MAIN OUTCOME MEASURES: The primary outcome was rate of nonattendance without cancellation. Secondary outcomes were cancellation and attendance rates and exploration of other factors associated with nonattendance. RESULTS: Patients (N=679) were allocated to receive either an SMS reminder (n=342) or no reminder (n=337). The nonattendance rate for patients who did not receive a reminder (16%) was more than nonattendance for patients receiving the SMS reminder (11%; odds ratio, 1.61; 95% confidence interval [CI], 1.03-2.51; number needed to treat, 19; 95% CI, 9-275). There were no differences in cancellation or attendance rates between groups. Exploration of other factors found that patients who were younger with a neck and trunk musculoskeletal or a neuromuscular disorder and who were scheduled to have an initial appointment or an appointment on a Monday or Friday were significantly predictive of increased nonattendance. CONCLUSIONS: SMS reminders can reduce nonattendance in physical therapy outpatient clinics.