Infrared television pupillometry was performed in 71 healthy and 66 insulin-treated diabetic subjects. Resting pupil diameter and the amplitude of the reflex response to standard light flashes were measured in background darkness. In the healthy subjects with small pupils (less than 6 mm) the size of the light reflex was found to depend on the diameter of the pupil in darkness, the former decreasing by 0.55 mm/mm decrease in diameter. A positive correlation was also found in the diabetic subjects with small pupils but the reflexes were much smaller for a given resting diameter than in healthy subjects. The majority of the diabetic patients with small pupils had autonomic neuropathy. It is suggested that impaired pupillary dilatation in diabetic subjects is due to a sympathetic neuropathy and the small light reflex response in some patients is due to parasympathetic dysfunction in addition.