The aims of the study are to analyse Swedish trends in overweight and obesity-as measured by body mass index (BMI)-between 1971 and 1995, and to examine socioeconomic and geographic differences between groups of individuals on the basis of information on 18-y-old military conscripts. The study population comprises all males born in 1953, 1958, 1963, 1968, and between 1973 and 1977, reported to be living in Sweden at 17 y of age according to nationwide population registers (RTPs). Utilizing the unique personal ID number, sociodemographic data in the RTPs and in Population and Housing censuses were linked to data on BMI in the national Military Service Conscription Registry for the years 1971 to 1995. These data were obtained from compulsory medical examinations held at military induction at 18 y of age. BMI data were available for 448 732 (89%) of a total of 503 689 subjects. Mean BMI increased by 6.6% over the study period-from 21.1 kg/m2 in 1971 to 22.4 kg/m2 in 1995. Unadjusted trend analyses showed a 2.4 times increase in the prevalence of overweight among 18-y-old males over the period-from 6.9% in 1971 to 16.3% in 1995. Over the same years, the prevalence of obesity increased 3.5 times-from 0.9% to 3.2%. After adjustment for muscle power, demographic factors, and living area, the prevalence of overweight was found to have increased 1.4 times, and obesity 1.7 times between 1971 and 1993. The prevalence of overweight was considerably higher among 18-y-old males from low-educated families than among those from high-educated families in both 1971 and 1995. The prevalence of overweight was also found to be higher in both 1971 and 1995 among young men from rural and/or sparsely populated areas than among those living in Sweden's largest cities. Mean BMI, and the prevalence of overweight and obesity have increased among 18-y-old males in Sweden over the last 24 y. The increased risk of overweight among young men from low-educated families and those from rural and/or sparsely populated areas detected in 1971 was still evident in 1995.