Clustering of selected coronary heart disease (CHD) risk factors was studied in a cohort of 3457 children and adolescents, aged 3-18 years. Subjects were defined as "high-risk" individuals if their values for serum LDL cholesterol, systolic blood pressure and obesity index (sum of biceps, triceps and subscapular skinfolds) exceeded the age- and gender-specific 75th percentile of the present study cohort. Clustering was studied by using the observed/expected ratio (O/E ratio). Statistically significant clustering was observed as 3.1% of all subjects belonged to the high-risk group (O/E ratio = 2.0, p < 0.001). Under the assumption of no association, only 1.56% would have been expected. Clustering was stronger in boys (3.6%) than in girls (2.6%). When other lipid variables (total cholesterol, HDL cholesterol, triglycerides) or obesity indices (subscapular, biceps, triceps skinfolds, the subscapular/triceps ratio and BMI) were used in the analyses, the degree of clustering remained essentially the same. The tendency of risk factors to cluster increased with age in boys, whereas in girls the strongest clustering was usually seen in the age group 9-12 years. When a fourth risk factor (low HDL cholesterol) was added to the analyses, the tendency of risk factors to occur simultaneously became even greater. Aggregation of multiple risk factors was strongest in boys who presented the highest levels of these risk factors. Common risk factors for CHD cluster in children and adolescents. The gender differences observed here may partly explain why males develop CHD more often than females in adulthood.(ABSTRACT TRUNCATED AT 250 WORDS)