A simple method for evaluating alterations in cardiac sympathetic innervation may be measurement of the Q-T interval. Seventy-three diabetic patients (46 insulin dependent and 27 non-insulin dependent) were separated into four groups based on the presence and degree of cardiac autonomic neuropathy (CAN) with noninvasive cardiovascular reflexes and blood pressure responses. None of the patients had evidence of ischemic heart disease, kidney disease, or the idiopathic long Q-T-interval syndrome. The corrected Q-T interval (Q-Tc) was determined at rest with Bazett's formula. As a group, diabetic patients with greater than or equal to 2 abnormalities of cardiac autonomic function had a longer Q-Tc interval than those with no evidence of CAN. Diabetic patients with greater than or equal to 1 abnormality had a prolonged Q-Tc interval compared with a control group of 96 healthy nondiabetic subjects (mean +/- SD 397 +/- 18). The frequency of prolonged (greater than 433 ms, normal mean + 2SD) resting Q-Tc intervals increased with the increasing number of abnormalities (0, 1, 2, greater than or equal to 3): 11, 25, 41, and 75%, respectively. Twenty-three of 25 (92%) patients with a Q-Tc greater than 433 ms had evidence of CAN. However, 57% (31 of 54) of the patients with CAN had a normal Q-Tc interval. These data provide further evidence of a relationship between the presence and severity of CAN and degree of Q-Tc interval prolongation.(ABSTRACT TRUNCATED AT 250 WORDS)