The aim of the study was to investigate relationships between physical fitness and self-reported physical activity in adulthood and to what extent the level of physical fitness and leisure-time physical activity in adulthood can be explained by anthropometric measures, physical performance, physical activity, attitudes to sports activities and socio-demographic characteristics at the age of 16. A group of 157 men and 121 women was tested at the ages of 16 and 34 by means of questionnaires and fitness tests. Physically active men and women had higher estimated VO2 max and performed better in curl ups and bench press than those who were inactive. Performance in physical tests, height, weight and physical activity at the age of 16 contributed best to explain adult physical performance and physical activity. The magnitude of explanation varied between 10% (9-min run test) and 56% (bench press test); it was in general lower in the men than in the women. The various fitness tests and physical activity were explained by different predictors and the predictors also differed between men and women. The findings about attitudes to sports and socio-demographic factors at a young age that influence adult physical activity habits and fitness are very complex and further research is required to identify specific inactivity risks.