The purpose of this study was to assess the effect of weight in middle-aged men and subsequent weight change on the risk of osteoporosis three decades later. The authors utilized data from 1,476 Norwegian men participating in two health screenings in Oslo (1972-1973 and 2000-2001) and Tromsø (1974-1975 and 2001). Height and weight were measured at baseline and follow-up. Total hip bone mineral density (BMD) was assessed at follow-up by dual energy x-ray absorptiometry. Baseline body mass index (BMI) was positively related to BMD three decades later. Subsequent weight change was also strongly related to BMD, and the proportion of persons with osteoporosis decreased from 15.1% among those who lost >or=10% of their body weight to 0.6% among those who gained >or=10% of their body weight. Excluding participants with medical conditions did not change the association between weight change and BMD. Taking both BMI and weight change into account, the prevalence of osteoporosis in the lowest quarter of baseline BMI was 31% (95% confidence interval: 24, 37) in persons losing >or=5% of their weight and 4% (95% confidence interval: 1, 7) in persons gaining >or=5% of their weight. In this cohort of middle-aged men, low baseline BMI and weight loss during the following three decades were both strongly and negatively related to total hip BMD.