Many studies have found an inverse association between fetal growth and cardiovascular disease related to the metabolic syndrome in adulthood. Nevertheless, the relative importance of genetics and the intrauterine environment remain unclear. The objective of the study was to test the fetal origins hypothesis and the fetal insulin resistance hypothesis by studying the impact of fetal growth on Body Mass Index (BMI) in young adulthood. In a nationwide cohort study, the Swedish Medical Birth Register for the years 1973–1979 was linked with the Military Service Conscription Register for 1990–1999. In 1998 a questionnaire was mailed to all male twins, included in the two registers, who were alive and still resident in Sweden. The study covers the 923 male twin pairs for which full data were available. Mixed linear models were used to estimate within-pair and between-pair differences in birthweight and their relations to BMI. A weak positive association was found among the monozygotic twins for the withinpair difference in birthweight and BMI. No significant association was found among the monozygotic for the between-pair difference in birthweight and BMI. No significant associations were found for dizygotic twins. These findings do not seem to support either the fetal programming hypothesis or the fetal insulin resistance hypothesis.