Cardiac autonomic function was measured in 25 subjects with insulin-dependent diabetes mellitus and 11 control subjects. Autonomic integrity was assessed with standard tests of autonomic function and a new technique of measuring heart-rate variability (HRV) for 24 h. All of the diabetic subjects were selected on the basis of peripheral or autonomic neuropathy or longterm poorly controlled diabetes. They were divided into groups according to presence or absence of vagal neuropathy based on the results of standard tests of autonomic function. Thirteen diabetic subjects had normal autonomic function tests (group 1), and vagal neuropathy was detected in 12 diabetic subjects (group 2). All subjects were monitored by ambulatory electrocardiograph, and the recordings were played back through an analyzer that identified and timed successive pulse (R-R) intervals. HRV was measured from the standard deviation of the successive differences between R-R intervals. HRV was significantly reduced in group 1 (mean ± SE 73 ± 9 ms) and group 2 (65 ± 12 ms) diabetic subjects compared with the control group (138 ± 10 ms). The standard tests of autonomic function did not distinguish the vagal dysfunction noted with HRV monitoring in group 1 diabetic subjects compared with control subjects. Measurement of 24-h HRV can detect small changes in cardiac autonomic function compared with currently available tests.