The classical risk factors, hypercholesterolemia, smoking, hypertension and diabetes, explain only a part of the epidemiological features of atherosclerotic coronary heart disease. Investigations in the past few years have shown involvement of immunological mechanisms in atherosclerosis. Circulating immune complexes accelerate atherosclerosis both in experimental animal models and in humans. The fourth component of complement (C4) plays an important role in the solubilisation and elimination of immune complexes. C4 consists of two allotypes, C4A and C4B. An earlier report showed an association between C4B null alleles (C4B*Q0) and myocardial infarction and to infarction related mortality. In the present investigation, C4A*Q0 and C4B*Q0 were studied in two population samples. The first (Group I) was a cross sectional study of 100 consecutive males with myocardial infarction before the age of 45 years and 164 population based healthy controls, age and sex matched. The second (Group II) was a nested case control study in which a cohort of 50 year-old males were followed for 20 years for development of myocardial infarction between 50-60 and 60-70 years, and the results compared with those who did not develop MI. We observed no association of homozygous and/or heterozygous C4A*Q0 or C4B*Q0 with myocardial infarction occurring in the age groups < 45, 50-60 and 60-70 years or with the infarction related mortality (P > 0.05). The prevalence/frequency of C4A*Q0 and C4B*Q0 was not related to the age at which MI occurred. The prevalence of C4A*Q0 was not affected by age. We thus conclude that partial deficiency of C4 does not appear to be a major risk factor for myocardial infarction.