Background: Among young adults, the metabolic syndrome is an increasingly frequent risk factor for cardiovascular disease. Its determinants are, however, incompletely understood. We investigated the time course, from adolescence (age, 13 years) to young adulthood (age, 36 years), of important potential determinants (body fatness and fat distribution, cardiopulmonary fitness, and lifestyle) in 364 individuals (189 women). Methods: Data were derived from the Amsterdam Growth and Health Longitudinal Study and analyzed with the use of generalized estimating equations. Results: The prevalence of the metabolic syndrome at the age of 36 years, as identified with a modified National Cholesterol Education Program definition of the syndrome, was 10.4%. Subjects with the metabolic syndrome at the age of 36 years, compared with those without the syndrome, had (from adolescence to the age of 36 years) the following: (1) a more marked increase in total body fatness and in subcutaneous trunk fat; (2) a more marked decrease in cardiopulmonary fitness levels; (3) a more marked increase in physical activities of light-to-moderate intensity, but a more marked decrease in hard physical activities; (4) a trend toward a higher energy intake throughout the years; and (5) a decreased likelihood of drinking alcoholic beverages. Conclusions: Fatness, fitness, and lifestyle are important determinants of the metabolic syndrome in young adults. More important, these associations were independent of each other and, therefore, represent separate potential targets for the prevention of the metabolic syndrome. Our study further suggests that intervening early in life (eg, in the period of transition from adolescence to adulthood) may be a fruitful area for prevention of the metabolic syndrome.