Sufficient evidence exists that plasma fibrinogen is a risk factor for cardiovascular disease to prompt a search for fibrinogen determinants. The authors measured fibrinogen in 4,193 participants in the Coronary Artery Risk Development in Young Adults (CARDIA) Study with three objectives: to describe fibrinogen levels in young adults, to determine if correlates of fibrinogen in older adults apply to younger adults, and to examine several additional hypothesized correlates. Fibrinogen (overall mean +/- standard deviation = 263 +/- 57 mg/dl) was greater in women than in men and in blacks than in whites; it was associated positively with cigarette smoking, body mass index, low density lipoprotein cholesterol, triglycerides, and blood pressure, and negatively with physical activity, high density lipoprotein cholesterol, and ethanol intake. Socioeconomic stress indicators and negative personality traits hypothesized as being associated with fibrinogen, possibly through adrenergic mechanisms, proved weak correlates. Parental history of diabetes was correlated positively with fibrinogen, in part because those with a positive parental history had greater body mass. Diabetes mellitus, lipoprotein(a), left ventricular mass, and oral contraceptive use were also positively correlated with fibrinogen concentration. Thus, even in young adults, fibrinogen is associated positively with most risk factors for atherosclerotic cardiovascular disease. This has implications for the role of fibrinogen as a risk factor and, potentially, for cardiovascular disease prevention.