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Twenty-five patients with extensive psoriasis were randomly assigned into one of the three groups, each receiving 0.5 mg/kg of oral liquid methoxsalen photochemotherapy followed 1 h later by exposure to long-wave ultraviolet light (UVA). The sole difference between the three groups was the method used to determine the initial UVA dose, which was either based on skin type, 25% of the minimal phototoxic dose (MPD), or 50% of the MPD. All patients were treated in the Phototherapy Unit at the Massachusetts General Hospital. Data were obtained until reaching the endpoint of clearance. At clearance, the results of the number of treatments required to clea, final UVA dose, cumulative UVA dose, and side effects were tabulated, compared, and analysed for each of the three groups. The 25% and 50% MPD groups required a mean of 15.0 ± 1.7% and 13.4 ± 1.9 treatments to clear, respectively, as compared to the skin type group, which needed an average of 17.6 ± 2.5 treatments. The mean final UVA dose was 7.4 ± 0.9 J/cm2 for the 25% and 50% MPD groups respectively, in contrast to 11.6 ± J/cm2 for the skin type group. The mean cumulative UVA dose at clearance for the 25% and 50% MPD groups was 79 ± 16 J/cm2 and 87 ± 27 J/cm2, respectively, versus 136 ± 30 J/cm2 for the skin type group. The comparisons between the individual MPD groups and the skin type group did not achieve statistical significance with the exception of a marginally significant difference in final dose between the skin type group and the 25% MPD group (p = 0.06). However, the results of the two MPD group were then pooled ant the mean final (7.9 ± 0.8 J/cm2) and cumulative (83 ± 15 J/cm2) UVA doses were significantly (p = 0.04) and marginally significantly (p = 0.07) lower than the respective means of the skin type group, did not attain statistical significance (p = 0.19). Our data suggest that the MPD measurement may be superior to the skin-typing system when calculating the initial UVA dose in oral liquid methoxsalen photochemotherapy for the treatment of psoriasis. [ABSTRACT FROM AUTHOR]