199 arthroscopies were performed on children with non-acute knee problems during 1985-1992 at the University Hospital of Oulu (76 boys and 123 girls). The mean age was 13 (2-15) years. The ratio between boys and girls was 1:1.6 showing the incidence among girls to be greater. The mean duration of symptoms was 16 (0.3-96) months before the arthroscopy. Major diagnostic groups were plica syndrome (43 cases), malalignment of the patella (29 cases), osteochondritis dissecans (28 cases) and arthritis of the knee (28 cases). The accuracy of clinical diagnosis was 47% in preadolescents and 51% in adolescents. Operations were performed arthroscopically in 71 cases, by open arthrotomy in 15 and by other extra-articular operations in 39. These children were followed until no further treatment was needed. The mean follow-up time was 4 years and 6 months (20-97 months). A total of 172 (87%) patients needed no further treatment, while 27 (13%) children were rearthroscopied and a reoperation was performed on 15 of these.