In 29 diabetic subjects with or without symptoms of diabetic peripheral and autonomic neuropathy, the association between the pupillary light reflex (the latency of the constriction of the pupil) and various sensory and motor neural measurements, duration of diabetes and quality of glycaemic control was evaluated. Compared with normal subjects the latency of the constriction reaction of the pupil was significantly prolonged in the group of diabetics (mean +/- SEM: 236.3 +/- 5.8 ms versus 210.5 +/- 3.7; P less than 0.001). Thermal discrimination thresholds were positively correlated with the latency of the constriction reaction of the pupil (r = 0.43, P less than 0.025). Vibratory perception threshold, motor nerve conduction velocity of the peroneal nerve, H-M interval of the Hoffmann reflex and duration of diabetes were not correlated with the constriction latency, whereas glycaemic control showed a weak correlation. It is concluded that there may be a relationship between autonomic (parasympathetic) pupillary dysfunction and peripheral small nerve fibre dysfunction.