Ten patients with hydroa vacciniforme are reported. The condition appears to be a distinct clinical and histological entity. Haematological, biochemical, immunological, bacteriological and viral investigations were negative. Three patients demonstrated low minimal erythema doses (MEDs) following monochromatic ultraviolet (UVA) irradiation of back skin; such UVA sensitivity may be a feature of hydroa vacciniforme. Treatment was unsatisfactory, although spontaneous improvement in the condition tended to occur and regular application of sunscreens with high protection factors against both UVA and UVB was helpful.