Plasma alpha-melanocyte stimulating hormone (alpha-MSH) concentrations were measured in patients receiving PUVA therapy as treatment for mycosis fungoides, and PUVA or UVB as treatment for psoriasis. Skin immunoreactive alpha-MSH was also measured in those patients who received PUVA. The mean plasma and skin alpha-MSH concentrations after 2-3 weeks of PUVA were not significantly different from pre-treatment values and showed no relationship either to skin type or to the degree of tanning that occurred in response to PUVA. Plasma alpha-MSH concentrations were also unchanged after UVB. There was also no short term change in plasma alpha-MSH concentrations in patients after receiving their first treatment with PUVA. It would appear that circulating and skin alpha-MSH levels are unaffected by UV and show no causal relationship to PUVA induced pigmentation.