The intrauterine environment seems to be important for the occurrence of cardiovascular and other chronic diseases in adulthood. The aim of the present study is to analyze the importance of birthweight, birthlength and Ponderal Index (PI) for Body Mass Index (BMI) and overweight at 18 years of age. In this cohort study, the nationwide Swedish Medical Birth Registry for the years 1973-1976 was linked with the national Military Service Conscription Registry for 1990-1996. Of 193,056 children born at term 1973-1976, 165,109 (85.5%) could be followed-up at 18 years of age. The children were divided into percentile groups according to birthweight and birthlength for each gestational week. They were also assigned to three different categories, according to birthweight and birthlength for each gestational week: infants with values < or = 5th percentile, values > 5th and < or = 95th percentile, and values > 95th percentile. Clear-cut associations were found between birthweight and BMI, and between PI at birth and BMI at 18 years of age. High birthweight and normal birthlength and also high birthweight and high birthlength for gestational age were found to be risk factors for overweight and severe overweight among young men in Sweden.