Objective: It is well established that overweight is related to osteoarthritis of the knees. The aim of this study was to investigate the risk of knee osteoarthritis for men and women in relation to body mass index (BMI) within the normal weight range and to assess the effect of former versus current weight. Methods: A population-based case-control study was carried out in the southern part of Sweden, including 825 cases with X-ray verified femorotibial osteoarthritis and 825 age-, sex-, and county-matched population controls. Mailed questionnaire data on weight, height, and confounding factors (heredity, smoking, knee injuries, and physical activity) were collected and analysed using logistic regression models. The response frequency was 89%. Results: Mean age of the participants was 63 years, and 57% were women. The adjusted risk of knee osteoarthritis was increased fourfold in men with a current BMI 23 to <25 kg/m2 as compared to men with BMI <23 kg/m2 (OR 4.0, 95% CI 1.7-9.5). The commensurate risk for women was 1.6 (95% CI 0.9-3.1). BMI at 30 years of age was similarly related to knee osteoarthritis. Conclusion: A moderate increase in BMI, within the normal weight range, was significantly related to knee osteoarthritis among men. Overweight at any time was related to knee osteoarthritis. © 2005 Taylor & Francis.