Margarine is a major source of trans fatty acids, the intake of which has risen since the early 20th century. Some data indicate that consumption of trans fatty acids increases the risk of coronary heart disease (CHD). In 1966-1969, 832 men from the Framingham Study, age 45-64 years and free of CHD, were administered a single 24-hour dietary recall, from which we estimated total daily margarine intake. We calculated CHD cumulative incidence rates and, using proportional hazards regression, CHD incidence rate ratios over 21 years of follow-up. Mean energy intake was 2,619 kcal; mean margarine intake was 1.8 (range 0-12) tsp per day. There were 267 incident cases of CHD. Age-adjusted CHD cumulative incidence rose over categories of margarine intake, but the increased risk was apparent only in the second half of the follow-up period. Adjusted for age and energy intake, the risk ratio for CHD for each increment of 1 teaspoon per day of margarine was 0.98 [95% confidence interval (CI) = 0.91-1.05] for the first 10 years of follow-up and 1.10 (95% CI = 1.04-1.17) for follow-up years 11-21. Adjustment for total fat intake and for cigarette smoking, glucose intolerance, left ventricular hypertrophy, body mass index, blood pressure, physical activity, and alcohol intake did not materially change the results. Butter intake did not predict CHD incidence. These data offer modest support to the hypothesis that margarine intake increases the risk of coronary heart disease.