The medial plica shelf syndrome as a clinical entity should be added to the list of differential diagnoses as a possible cause of symptoms of internal derangement of the knee in children. It most frequently mimics chondromalacia of the patella and indeed may lead to it. It can be clinically suspected in the presence of the signs described in this report. The condition can be diagnosed early and accurately by arthroscopy and thereafter treated satisfactorily, with minimal morbidity. The results of such treatment not only have been gratifying over the period of follow-up (five to 32 months), but in all probability have prevented further serious damage to the knee.