In a case-control study, body-mass index (BMI) at aged 20, 30, 40, and 50 years was studied in 239 men who had just received a hip prosthesis because of coxarthrosis and in 302 controls randomly selected from the general population. Information about weight, occupational physical load, sports activities, smoking, and many other factors was self-reported in a questionnaire. Men with a BMI > the mean BMI + 1 SD had an increased relative risk of developing severe coxarthrosis as compared with men with a BMI < the mean BMI -1 SD. Those slightly obese at the aged 40 years had a relative risk of 2.5 for later surgery of the hip.