We examined 320 patients with MRI and arthroscopy after an acute trauma to evaluate MRI in diagnosis of degenerative joint disease of the knee in relation to sports activity and clinical data. Lesions of cartilage and menisci on MRI were registered by two radiologists in consensus without knowledge of arthroscopy. Arthroscopy demonstrated grade-1 to grade-4 lesions of cartilage on 729 of 1920 joint surfaces of 320 knees, and MRI diagnosed 14% of grade-1, 32% of grade-2, 94% of grade-3, and 100% of grade-4 lesions. Arthroscopy explored 1280 meniscal areas and showed degenerations in 10%, tears in 11.4%, and complex lesions in 9.2%. Magnetic resonance imaging was in agreement with arthroscopy in 81% showing more degenerations but less tears of menisci than arthroscopy. Using a global system for grading the total damage of the knee joint into none, mild, moderate, or severe changes, agreement between arthroscopy and MRI was found in 82%. Magnetic resonance imaging and arthroscopy showed coherently that degree of degenerative joint changes was significantly correlated to patient age or previous knee trauma. Patients over 40 years had moderate to severe changes on MRI in 45% and patients under 30 years in only 22%. Knee joints with a history of trauma without complete structural or functional reconstitution showed marked changes on MRI in 57%, whereas stable joints without such alterations had degenerative changes in only 26%. There was no correlation of degenerative disease to gender, weight, type, frequency, and intensity of sports activity. Therefore, MRI is an effective non-invasive imaging method for exact localization and quantification of chronic joint changes of cartilage and menisci that recommends MRI for monitoring in sports medicine.