A properly performed thermoregulatory sweat test (TST) can be informative in patients with diabetic neuropathy. Of 51 patients suspected of having neuropathy on the basis of a clinical examination, 48 (94%) had unequivocal abnormalities on the TST. Pathologic loss of sweating occurred distally in 65%, segmentally in 25%, and only in isolated dermatomes in 25%; 78% of patients had a combination of two or more patterns. Global anhidrosis was noted in eight patients (16%), all of whom had profound autonomic neuropathy, and in the entire group, the percentage of body surface anhidrosis correlated with the degree of clinical dysautonomia (rank correlation coefficient = 0.77; P less than 0.01). Discrete zones of anhidrosis on the thorax and abdomen were noted in patients with painful diabetic radiculopathy, and they correlated highly with thoracic paraspinal muscle fibrillation potentials. Distal loss of sweating detected on the TST was always associated with a subnormal quantitative sudomotor axon reflex response or abnormal electromyographic findings, an indication of a distal axonal neuropathy. The TST provides reliable information about the distribution of diabetic neuropathic involvement and can be especially useful in the diagnosis of truncal radiculopathy and clinically significant autonomic neuropathy.