The relationship between whole blood selenium levels and risk of acute myocardial infarction was investigated in a community-based control study in Auckland, New Zealand. A pilot study in 14 patients admitted to hospital within 4 hours of onset of symptoms demonstrated that selenium levels were stable in the first 16 hours after admission for an acute myocardial infarction. Some 252 cases (199 men, 53 women) presenting to hospital within 20 hours of onset of acute myocardial infarction were compared with 838 controls (500 men, 338 women), group-matched for age and sex. Myocardial infarction patients had significantly lower mean selenium levels: 82.8 and 87.9 micrograms/l in male cases and controls (p = 0.003) and 82.1 and 88.5 micrograms/l in female cases and controls (p = 0.02) respectively. The relative risks of myocardial infarction in participants with selenium levels below the median level (85 micrograms/l) in comparison with participants above the median were 1.6 (95% CL 1.1-2.2) and 1.7 (95% CL 0.9-3.5) in men and women respectively. The effects of a low selenium level on risk of myocardial infarction were confined to cigarette smokers. These results suggest the hypothesis that a decreased blood selenium in the presence of cigarette smoking is a risk factor for coronary heart disease.