In children, compulsory arthroscopy for hemarthrosis after knee trauma is not justified because ligamentous and meniscal damage is rare. In a prospective study, we analyzed the diagnostic value of radiography, magnetic resonance imaging (MRI), and arthroscopy in 51 patients up to 14 years of age with acute knee trauma. Plain radiography revealed 16 osseous lesions (5 metaphyseal, 3 patellar, 4 physeal fractures, 3 avulsions of the tibial spine, and 1 osseous ligamentous tear). In 29 patients, the cause of hemarthrosis remained unclear. All patients were evaluated by MRI. A diagnosis could be assigned to all 29 patients. MRI demonstrated lesions in 38 patients. In addition, the following lesions were discovered: 8 patellar dislocations, 13 bruises, 1 rupture of the anterior cruciate ligament, 1 osteochondritis dissecans, and 13 joint effusions. In 13 patients, MRI was followed by arthroscopy to confirm the diagnosis. Both, MRI and arthroscopy missed two osteochondral fractures. In addition, three chondral lesions were not picked up by MRI. MRI is a reliable tool for assessing the extent of knee lesions in children.