The variation in erythemal sensitivity of the skin during PUVA therapy with oral 8-methoxypsoralen (8-MOP) was examined by measuring both UVA and PUVA erythemal responses, together with plasma 8-MOP concentration, in 27 patients about to start PUVA therapy for psoriasis. The erythema responses were judged visually, and also measured using a reflectance instrument in order to construct dose-response curves. No significant association was found between the UVA and PUVA minimal erythema responses. The plasma psoralen concentration showed significant association with the slope of the PUVA erythema dose-response curve. The slopes of the UVA and PUVA erythema dose-response curves were significantly associated, and this association became much stronger when allowance was made for plasma psoralen concentration. These results show that erythemal sensitivity during PUVA therapy is related to both plasma psoralen concentration and inherent UVA sensitivity, but that this relationship is not apparent when sensitivity is judged visually as the minimal erythema response. The association between PUVA and UVA erythemal sensitivity suggests a common pathway in the vascular response induced by UVA radiation, with or without psoralen.