Prior studies of the contribution of coronary disease risk factors to familial aggregation of premature coronary disease may have underestimated risk factors by relying on self-reported risk factor prevalence levels or, when risk factors have been measured, by using cut points in excess of the 90th percentile. To determine the actual prevalence of hyperlipidemia, hypertension and diabetes, and the awareness of these coronary risk factors in unaffected family members, 150 apparently coronary disease-free siblings of 86 people who had documented coronary disease before 60 years of age were studied. All subjects participated in a 1 day screening preceded by a self-administered risk factor questionnaire and a personal interview. Participation of both the index patients and siblings exceeded 86%. With the use of nationally established recommendations for blood pressure and lipids, which are based on coronary disease risk curves, screening revealed that 48% of brothers and 41% of sisters were hypertensive, 45% of brothers and 22% of sisters had a lipid abnormality, 38% of siblings were current cigarette smokers and 4.7% were diabetic. Two or more risk factors were present in 42% of brothers and 26% of sisters. More than 75% of siblings had one or more risk factors that would require intervention. When compared with a race-, gender- and age-matched reference population from the Lipid Research Clinics Prevalence Study, distributions for blood pressure and for total and low density lipoprotein cholesterol were higher for the siblings in every gender and age group.(ABSTRACT TRUNCATED AT 250 WORDS)