The classic randomized clinical trial is difficult to apply to the evaluation of surgical procedures. After reviewing the structure and rationale of cohort studies and randomized clinical trials, four types of bias in randomized clinical trials that present difficulties in clinical studies are identified. Other problems that may limit the willingness of orthopaedic surgeons to conduct randomized clinical trials relate to the surgical procedure itself, the outcome of the trial of interest, surgical philosophy, and the ethical implications for a surgeon of a randomized clinical trial. A recently proposed variation of the classic randomized clinical-trial model, the randomized-surgeon design, appears to solve some of these problems.