Fifty-one patients with psoriasis were treated with oral methoxsalen and sunlight exposure. Twelve of these patients received either methoxsalen or placebo prior to whole-body exposure. The remainder were treated with methoxsalen and sunlight to one side of the body and sunlight alone to the other. The conventional dose of methoxsalen (0.6 mg/kg) was compared with a low dose (0.3 mg/kg). Oral methoxsalen when used in the higher dose followed by sun exposure is an effective treatment for psoriasis. Accurate ultraviolet dosimetry is essential to avoid phototoxic burns. The advantages and disadvantages of solar photochemotherapy are discussed.