A prospective, randomized study was performed to compare the results of arthroscopic lateral retinacular release (Group I) with those of open lateral retinacular lengthening (Group II). The study included eighty-six patients (eighty-six knees) who had had pain in the anterior peripatellar aspect of the knee and lateral patellar tilting (rotational malalignment), as seen on Merchant tangential patellofemoral radiographs, that had not improved after participation in a structured rehabilitation program for the quadriceps and the hamstrings for a minimum of six months. The mean duration of follow-up after the operative procedures was forty-six months (range, two to six years).At the time of the latest follow-up evaluation, forty (93 per cent) of the forty-three patients in Group I and all forty-three (100 per cent) of the patients in Group II had resumed the level of athletic activity that they had engaged in before the onset of symptoms (p = 0.08, analysis of variance). With the numbers available for study, no significant difference could be detected between the groups with regard to deficits in the range of motion, atrophy (the circumference) of the thigh, operative or postoperative complications, or the need for subsequent operative procedures. Additionally, no significant difference could be detected between the groups with regard to the results of open or closed-chain testing with an isokinetic dynamometer.According to the knee-rating system of Lysholm and Gillquist as modified by Tegner and Lysholm, thirty-three knees (77 per cent) in Group I had a score of 80 points or more compared with thirty-eight knees (88 per cent) in Group II. Six knees (14 per cent) in Group I had a score of less than 70 points compared with no knees in Group II. The difference in the knee ratings between the two groups was significant (p = 0.028, analysis of variance).Although there seemed to be a definite trend toward improved function of the knee in association with a longer duration of follow-up, no significant association could be detected between the duration of follow-up and improvement in the outcome measures in either group.