Hyperapobetalipoproteinemia (hyperapoB) is one of the most common phenotypes in patients with premature coronary heart disease. In this study the factors that affect the expression of the hyperapoB phenotype were evaluated in young individuals. A cohort of 1125 children and young adults aged 9-24 years was classified into three groups by sex: (1) normal serum apolipoprotein B (apoB), (2) high apoB (> or = 90th percentile) and normal low density lipoprotein cholesterol (LDL-C < 90th), (3) high apoB and high LDL-C (> or = 90th percentile). In females, alcohol use (11, 33, 0%, in groups 1-3, P < 0.05) and oral contraceptive use (35, 83, 47%, P < 0.01) were significantly different between the groups and the highest frequencies were seen in the hyperapoB group (group 2). In both sexes smoking tended to be more common in the hyperapoB group (29, 43, 18%, P < 0.14). The two hyperapoB definition criteria (high apoB and low LDL-C/apoB ratio) were studied with multiple linear regression analyses. Oral contraceptive use correlated positively with apoB values (coefficient beta = 0.101, R2 = 2.1%, P < 0.01) and negatively with LDL-C/apoB ratio (beta = -0.134, R2 = 3.3%, P < 0.001). Alcohol use (beta = -0.072, R2 = 2.9%, P < 0.001) and smoking (beta = -0.050, R2 = 1.0%, P < 0.05) correlated negatively with LDL-C/apoB ratio. Prevalence of the hyperapoB phenotype was 4.4%. According to the results, the expression of the hyperapoB phenotype may be influenced by common lifestyle habits. This should be considered if high risk young individuals are identified through the expression of the hyperapoB phenotype.