In a prospective two-year study on urban (city of Helsinki) Finnish children, 72 acute patellar dislocations were observed. The calculated annual incidence rate was 43/100,000 in children under 16 years. A total of 28 knees (39%) had associated osteochondral fractures. These fractures comprised 15 capsular avulsions of the medial patellar margin and 15 loose intra-articular fragments detached from the patella and/or lateral femoral condyle. The intra-articular fragments were found only after spontaneous relocation of the patella. The femoral fracture constantly involved the edge of the articular surface in the middle third of the condylar arc.