Five patients with actinic prurigo were treated twice weekly with PUVA. One area on the back was shielded from UVA throughout the 15-week treatment period. Before PUVA, all patients had increased erythemal sensitivity to UVA and showed abnormal augmentation of UVB erythema by topical indomethacin. After PUVA, all patients were free of photosensitive symptoms and skin that had been exposed to UVA showed normal erythemal responses. By contrast, the areas of skin that had been protected from UVA showed erythemal responses that were unchanged from pre-PUVA values. Augmentation of UVB erythema by topical indomethacin persisted, both on UVA exposed and UVA protected skin. These results show that, although PUVA is an effective treatment in actinic prurigo, it does not alter the underlying mechanism of photosensitivity. The protective effect is local and is due presumably to an increase in melanin pigmentation and epidermal thickness.