The Multiple Risk Factor Intervention Trial (MRFIT) was one of the coronary heart disease prevention trials recommended to the National Heart and Lung Institute in 1971 as an alternative to a national single-factor dietary trial, which was judged to be infeasible. MRFIT was a randomized, primary prevention trial, conducted at 22 US clinical centers from 1973 to 1982 to test whether lowering elevated serum cholesterol and diastolic blood pressure and ceasing cigarette smoking would reduce coronary heart disease mortality. Men 35-57 y of age (n = 12,866) with one or more of these risk factors were randomly assigned to the special intervention (SI) or usual care (UC) group and followed for 6-8 y. UC men were given information on risk factors, referred to their usual sources of care, and reexamined annually. SI participants received group and individual counseling on a fat-modified diet, a stepped-care drug treatment program for diastolic hypertension (after an initial attempt at blood pressure control by weight reduction, if indicated), and, for cigarette smokers, counseling aimed at cessation. SI men had risk factor assessments every 4 mo and annual examinations that were generally identical to those given to UC men and that always included measurement of blood cholesterol concentration. A listing of variables measured at each visit along with the design and major mortality results of MRFIT are included in this chapter.