Arthroscopy of the knee was performed in 104 patients under 18 years of age, 41 in children (aged 8 to 15) and 63 in adolescents (aged 16 to 18). Most frequently lesions of the patella were found (in 45% of all diagnoses in children and 29% in adolescents), in most cases because of acute or recurrent dislocation of the patella. The incidence of meniscal lesions increased with age. The most frequent therapeutic procedure performed in children was a lateral release (34%), in adolescents a partial meniscal resection (31%). In children 43% of arthroscopies were diagnostic, in adolescents 21%. Preoperative diagnosis was shown to be incorrect or incomplete arthroscopy in 41% of children and 24% of adolescents taking arthroscopic findings as a golden standard. Preoperative diagnosis had highest accuracy for dislocation of the patella, 90% of dislocations were diagnosed correctly before arthroscopy. The lowest accuracy of clinical diagnosis was found for meniscal lesions; only 36% of meniscal tears were suspected preoperatively. In 35 of 38 patients with hemarthrosis a relevant knee lesion was found, in 23 of these patients arthroscopic therapy was performed. This justifies our concept for arthroscopy of every knee hemarthrosis. Besides superficial chondral lesions in 8 patients there were no complications associated with the procedure. It is concluded that arthroscopy of the knee in children and adolescents is a safe procedure with high diagnostic and therapeutic value.