AIMS/BACKGROUND--Pupillary light response is usually defective in all types of optic neuropathy. However, the authors have observed in patients with Leber's hereditary optic neuropathy (LHON) relatively normal light response, with consequent misdiagnosis psychogenic visual loss in some cases. To confirm this clinical impression, afferent pupillary defect was assessed by measurement of adjusted constriction amplitude (CA) and escape rate (ER) by infrared videopupillography (Iriscorder-C 2515). METHODS--Thirteen consecutive patients (26 eyes) with LHON (average age 27.2 years) were examined; 12 had the mitochondrial DNA 11778 mutation and one the 14484 mutation. Seven of these patients had a positive family history. For comparison, the above rates were determined in 19 patients (23 eyes) with idiopathic optic neuritis (ON; average age 35.1 years), 18 patients (19 eyes) with anterior ischaemic optic neuropathy (AION; average age 58.1 years), and 25 volunteers (50 eyes) with healthy eyes (average age 39.6 years). RESULTS--The distribution of visual acuity was essentially the same in all optic neuropathy groups. Reduction in CA and increase in ER were significant in patients with ON and AION, but not in those with LHON. Only slight afferent pupillary defect was evident even 2 years after the onset of LHON. CA in AION and ER in ON were correlated statistically with visual acuity and Humphrey mean threshold deviation, while CA and ER in LHON were not. CONCLUSION--Pupillary light response in patients with LHON obviously differs from that in patients with other types of optic neuropathy. LHON appears to be pathophysiologically characterised by well preserved afferent fibres for pupillary light response (probably from W cells). Besides being of pathogenetic interest, the detection of clinical features should facilitate the diagnosis of LHON particularly when family history provides no indication.