Moderate (greater than 20 units) and high (greater than 80 units) IgG anticardiolipin antibody (aCL) titres are strongly predictive for recurrent thrombosis and early myocardial infarction in non-diabetic subjects. We have tested the hypothesis that the excess risk of myocardial infarction in diabetic subjects relates to the presence of aCL by measuring the frequency and titre of aCL in two groups of diabetic subjects and in 2500 healthy controls. One non-diabetic subject (0.04%) had low (5-20 units) IgG aCL titres. Seven out of 126 diabetics without cardiovascular disease (5.6%) and 9 out of 79 diabetics who were either myocardial infarction survivors or who had angiographically-proven coronary artery disease (11.4%) had low aCL titres (P less than 0.01 for comparison of either diabetic group with controls, and P less than 0.1 for comparison between diabetic groups). One subject in each diabetic group, but no non-diabetics, had moderate IgM aCL titres. No subjects had high aCL titres. Diabetics have an increased frequency of low aCL titres which may relate to macrovascular disease. Macrovascular disease in diabetics is not associated with moderate or high aCL titres.