A 10-year follow-up study was carried out among 6647 farmers aged 40-64 y to investigate associations between age, gender, body mass index (weight/height2) and the incidence of disabling knee osteoarthritis. After the follow-up (1980-1990) a record linkage with the Social Insurance Institution's register was made to identify disability pensions. All the medical certificates for the disability pensions (N = 965) were reviewed to find out the incident cases (N = 126). Age, female gender and body mass index were all independent predictors of disabling knee osteoarthritis. The relationship between body mass index and the incidence of disabling knee osteoarthritis was linear, the adjusted relative risk was 1.4 (95% CI, 1.2-1.5) per standard deviation of the index (3.8 kg/m2). In women, as compared with men, the adjusted relative risks (95% confidence intervals) of right unilateral, left unilateral and bilateral knee osteoarthritis were 7.0 (2.5-19.7), 3.3 (1.1-9.8) and 4.8 (2.4-9.3), respectively. Overweight is a strong risk factor for disabling knee osteoarthritis.