A high level of plasma homocyst(e)ine (H(e)) has been reported to be an independent risk factor for coronary heart disease (CHD), at least in some populations. We have determined the H(e) concentration in the plasma of two series of Norwegians in order to establish a baseline for future analysis of people with CHD. The mean sex- and age-adjusted homocyst(e)ine level was 10.6 (range 4.84-29.88) in one series and 10.5 (range 3.76-40.57) in the other. The H(e) level appeared to be independent of other proven or potential risk factors or protective factors with respect to CHD. The intraclass correlation coefficient in monozygotic (MZ) twins is a (possibly inflated) estimate of heritability. We have examined two series of MZ twins. The intraclass correlation coefficient was significant in both series. In one series, the Pearson correlation coefficient was 0.53 and the Kendall correlation coefficient 0.38. In the other, the values were 0.56 and 0.46, respectively. We conclude that in the population examined, H(e) levels exhibit significant heritability.