One patient with urticaria pigmentosa was treated with PUVA. This treatment resulted in generalized tanning, decrease of Darier's sign and improvement of pruritus. However, the hyperpigmented macules overlying the mast cell infiltrates showed some persistence. This patient was subjected to biopsy of lesional skin before, during and at the end of oral photochemotherapy. During and after PUVA-therapy, histological examination revealed an increased number of free granules and a progressive decrease in the number of dermal mastocytes. Ultrastructurally necrotic cellular changes in the dermal mastocytes were observed. This suggests that PUVA-therapy may exercise its beneficial effect in urticaria pigmentosa by direct destruction of these cells. In addition, PUVA stimulates mast cell degranulation. Histaminaemia was estimated in this patient, before and after several irradiations. No significant changes were observed. Furthermore, histaminaemia was assayed in 15 patients with various dermatoses undergoing PUVA-therapy. No significant changes were observed. It is concluded that histaminaemia assayed using the fluorometric method of Shore et al. remains unchanged during oral photochemotherapy.