The objective of this study was to test the hypothesis that excessive severity of ischaemic heart disease in diabetics is due, in part, to capillary inadequacy. Sections from autopsied hearts of diabetic patients with and without myocardial infarction as well as from those of patients with infarcts and no diabetes were used for morphometric studies of intramural microvessels in areas without infarction. Normoglycaemic patients with normal hearts were also examined. Two to five transverse sections from each of 44 hearts (stained with methenamine silver) were examined for capillary numerical density, capillary to myofibre ratios, and myofibre diameters. Averages for each case and totals for each group were calculated and compared. Normoglycaemic patients with infarcts had increased morphometric values. Diabetics with infarcts had significantly lower capillary densities than the other groups. In conclusion, it is suggested that in diabetes there is an inadequate ischaemia-induced, reactive angiogenesis. This may contribute towards increased myocardial vulnerability in further ischaemic injury and perhaps to diabetic cardiomyopathy.