Clinical and arthroscopic findings after knee joint injury were evaluated in 35 children aged 4-15 years. Arthroscopy revealed a rupture of the anterior cruciate ligament in 9 patients; two lesions were only partial. Eight children suffered from patellar dislocation. Eleven patients showed minimal synovial and cartilaginous lesions as the only cause of hemarthrosis, hydrarthrosis and clinical symptoms. Three had chondral lesions due to a dysplastic patella. The clinical diagnosis was confirmed in only 12 of 35 cases. Arthroscopic therapy was performed in 15 and open surgical intervention in 16 patients. Our results show that hemarthrosis and persisting symptoms after a knee trauma in children indicate a severe knee injury. The diagnostic procedure and treatment should be as ambitious as in adults.