The relationship between sports activities and knee osteoarthrosis (OA) in the general population was investigated. Men (n=325) and women (n-=300) who underwent prosthetic surgery due to primary tibio-femoral OA were compared with referents (264 men and 284 women) regarding life-long sports exposure. The relative risk of severe knee OA among men aged between 55 and 65 years who were highly exposed to all kinds of sports was 2.9 (95% CI 1.3-6.5). For sports such as cross-country skiing, soccer and ice hockey/bandy, the relative risk was higher for those men who had been exposed. The women had only been active in sports to a limited extent, and no increased relative risk was found. Moderate daily general physical activity was not found to be a risk factor.