To determine the number of patients needed to be screened (NNS) and allocated (NNA) in order to include one participant in a randomized clinical trial (RCT), and to compare the characteristics of patients accepting or declining participation in the RCT.The recruitment process of an ongoing multicenter RCT, comparing surgical and non-surgical interventions after acute anterior cruciate ligament (ACL) injury of the knee is described. We use the known concept Number Needed to Screen (NNS) and introduce the new concept Number Needed to Allocate (NNA) as variables to support a priori sample size calculations of future investigations.560 patients were screened to identify 162 patients (29%) eligible for inclusion in the RCT. 41 of those declined participation for various reasons, the most common being unwillingness to undergo surgery (n=23) or unwillingness to risk conservative treatment (n=8). 19 patients were excluded after MRI assessment or arthroscopy. Thus, 102 (18%) patients were allocated to one of the two treatments in the RCT. The NNS was 5.5 individuals with an acute knee injury, and the NNA was 1.6 individuals eligible for inclusion, to include 1 patient in the RCT. Patients declining to participate in the RCT were more frequently self-employed and less frequently injured in sports activities than those accepting RCT participation.We suggest that the a priori sample size calculation needs to be multiplied by at least 5.5 to provide an estimate of the number needed to screen, or 1.6 to provide an estimate of the number needed to allocate in order to include the desired number of patients in a trial comparing surgical and non-surgical treatment of the ACL injured patient.