Pupil cycle time (PCT) is an ophthalmologic technique that measures the speed of nerve conduction throughout the pupil reflex arc. An abnormally prolonged measurement may reveal subtle lesions to the autonomic nervous system. This procedure was applied to the detection of lesions in 10 experienced professional divers and three control groups: 20 non-divers, 20 sport divers, and 10 trainee professional divers. The professional group demonstrated a prolonged mean pupil cycle time compared to other groups (F = 9.732, P < 0.001). Professional divers were significantly older than other subjects, reporting longer diving histories and a higher incidence of decompression sickness. Only professional divers had saturation or trimix experience. Two divers reported long-term visual or cognitive disturbances, but all subjects demonstrated normal visual acuity. Multiple regression analysis indicated that one variable had a predominant (P < 0.01) effect on PCT: the maximum depth attained using mixed gas breathing mixtures. The utility of the technique as a component of long-term neurologic examination is discussed.