Diagnostic arthroscopy was performed in 40 consecutive patients under 16 years of age with hemarthrosis of the knee. The age of the patients ranged from 10 to 15 years, median 13.7 years. Eighteen patients had ruptures of the anterior cruciate ligament; seven ruptures were partial. Only two cases of fractures of the tibial spine were encountered. Seven patients had midsubstance tears of the anterior cruciate ligament. Eleven patients had patellar dislocations. Six patients had minimal capsular disruptions as the cause of hemarthrosis. Three patients had osteochondritis dissecans of the knee and two patients had isolated ruptures of the medial collateral ligament. Twenty-eight of the patients underwent some form of operative repair. The clinical diagnosis was confirmed at arthroscopy in 17 of the 40 cases. The appearance of hemarthrosis of the knee usually signifies a serious knee injury and should be treated with the same importance as in adults, and as a rule arthroscopy should be carried out.