Some of the characteristics of men, with known risk factors, who were most likely to respond to an invitation to be screened in a national randomized clinical trial to prevent heart disease were determined in 18,872 men, 35-57 years of age, members of the Kaiser Foundation Health Plan of Oregon. Demographic characteristics and risk factor variables (blood pressure, blood cholesterol, and cigarette smoking levels) were abstracted from medical records.The men were ranked high priority or low priority according to level of risk. All age-eligible men in the health plan received at least one invitation to be screened, with high-priority men receiving more in-