In an attempt to improve the prediction of PUVA erythemal sensitivity, we have examined, in 2 51 patients, the relationship between the minimal phototoxic dose (MPD) and a number of variables, including skin type, ingested dose of 8-methoxypsoralen (8-MOP) and history of previous PUVA treatment. The MPD was determined by phototesting 2 h after ingestion of crystalline 8-MOP. given at a standard dose of 0–6 mg/kg (calculated to the nearest 10 mg). No reaction to the highest dose of UVA used for phototesting occurred in 16% of cases; the dose of 8- MOP was significantly associated with non-response. In the patients in whom an erythemal response was obtained, a significant association was found between the MPD and the variables of skin type, ingested dose of 8-MOP. and history of previous PUVA treatment. Thus, by taking these factors into account, a more accurate prediction can be made of an individual patient’s erythemal sensitivity to PUVA than relying on skin type alone. That erythemal sensitivity is affected by the ingested dose of 8-MOP (even when all patients received 0–6 mg/kg) suggests that conventional psoralen dosing according to body weight is not ideal.