Autonomic neuropathy is associated with an increased incidence of silent myocardial infarction and sudden death. The purpose of this study was to investigate the prevalence of silent myocardial ischaemia in diabetic patients with autonomic neuropathy and without. Five standard autonomic function tests were performed on 41 men with diabetes: postural change in blood pressure, postural change in heart rate, heart rate response to deep breathing, heart rate response to Valsalva's manoeuvre, and blood pressure response to sustained handgrip. There were 17 patients with autonomic neuropathy (group A) and 24 with normal autonomic function (group B). All patients underwent 24 hour ambulatory electrocardiographic monitoring to detect silent ischaemia. There was no significant difference in risk factors for coronary artery disease or history of angina pectoris between these groups. The prevalence of silent ischaemia was 64.7% in group A (95% confidence interval (95% CI) 38.33 to 85.79%) and 4.1% in group B (95% CI 0.11 to 21.12%). This represents a relative risk of 42.2 (95% CI 4.5 to 39.4, p less than 0.001). These results are consistent with the concept that autonomic neuropathy may prevent the development of anginal pain and thus obscure the presence of ischaemic heart disease. Twenty four hour ambulatory electrocardiographic monitoring may identify a subgroup of diabetic patients with autonomic neuropathy who have myocardial ischaemia and to whom treatment may be offered.