The aim of this study was to investigate gender-related differences in physical activity patterns at the ages of 16 and 34 and to analyse those factors that might contribute to an explanation of physical activity habits in adulthood. A randomly selected group, consisting of 220 boys and 205 girls, was tested in 1974 and reinvestigated in 1992 by means of a questionnaire. Eighty-eight per cent participated in the reinvestigation. At the age of 16 more boys (69%) than girls (51%) participated in some sports activity in their leisure time. The results showed an overall decrease in participation in vigorous physical activity, although participation in light physical activity remained relatively constant. At the age of 34, there was no difference in overall physical activity between men and women, but the men exercised more vigorously (44% vs. 29%). Early experience of physical activity at the age of 16 decreased the risk of becoming inactive in adulthood. At the age of 34 cohabiting for the men, and having children and high socio-economic class for the women, increased the risk of being physically inactive in adulthood, whereas positive beliefs about health effects of exercise decreased the risk for both men and women.