A series of 21 patients admitted to St Thomas' Hospital, Medical Ophthalmology Unit, with a diagnosis of West Indian or West African amblyopia is reported. Patients were investigated for haematological, biochemical, serological, and radiological abnormalities and particular attention was paid to dietary history. Patients admitted in recent years also underwent neurophysiological investigations. No definite correlation between visual loss and dietary or family history was found, and there was no evidence that the improvement in vision which occurred in just under half the patients on follow-up was related to treatment with hydroxocobalamin or multivitamins. Visual-evoked responses in 4 patients showed a prolonged latency suggesting optic nerve demyelination, while in only one case was the electro-oculogram definitely subnormal. These findings contrast with those in 'toxic' amblyopias and suggest that the syndrome of West Indian amblyopoa may be due to bilateral optic nerve demyelination of unknown aetiology rather than the effect of toxic substances or nutritional deficiency on the retina.