The Multiple Risk Factor Intervention Trial was a randomized primary prevention trial to test the effect of a multifactor intervention program on mortality from coronary heart disease in 12,866 high-risk men aged 35 to 57. The primary screening for MRFIT began in November 1973 and continued through November 1, 1975. The primary screening was conducted at clinical center sites, central neighborhood locations, and places of employment. Information was collected on smoking history, blood pressure, serum cholesterol, birthdate, race, and social security number as well as previous hospitalization for a heart attack and use of medication for diabetes. Systolic and diastolic blood pressures were measured and blood drawn for serum cholesterol determinations. The results of the screening examination were transmitted to the Coordinating Center.