Seven patients with diffuse lichen planus were treated with oral photochemotherapy. Remission was effected in 6 cases. The only failure may be attributable to a low UVA dose. Histological examination after treatment showed disappearance of the superficial dermal lymphocytic infiltrate. Ultrastructural study revealed nuclear alterations of keratinocytes and nuclear and cytoplasmic changes in superficial dermal cells. Various theories concerning the mode of action of photochemotherapy in lichen planus are discussed in the light of these observations.