The presence of autoantibodies to autonomic nervous tissue structures is a feature of patients with symptomatic diabetic autonomic neuropathy. It has not been established whether these autoantibodies cause, contribute to or simply reflect nervous tissue damage. Serum samples were tested for the presence of complement-fixing autoantibodies to adrenal medulla, vagus nerve, and sympathetic ganglion cells, to demonstrate: (a) reproducibility of the technique, (b) persistence of the antibodies, and (c) whether or not they occur in patients with non-insulin-dependent (Type 2) diabetes (NIDDM) with neuropathy. Examination of 37 samples, by different observers 2 years apart, revealed a high degree of concordance of both positive and negative results, demonstrating the method of testing to be highly reproducible. Re-testing of 37 patients (by analysing a second blood sample) between 0.5 and 2.7 years (mean 1.7 years) after their first test also demonstrated that antibodies, once present, normally persist; and that most patients initially negative remained so. Of 17 neuropathic NIDDM patients, 16 were negative for all three antibodies, indicating their rarity in this group of patients.