The coexistence of psoriasis and lichen planus has been rarely reported, although both diseases are relatively common and have some features in common. A rare case of simultaneous occurrence of both diseases is presented. Spontaneous improvement of psoriasis occurred during the development of lichen planus and a subsequent disappearance of the lichen planus was associated with an exacerbation of the psoriasis. The clinical courses of the two diseases may not be independent of each other, but be affected by common factors. Immunohistochemical studies with lesional skin suggested local production of interferon-γ, which may in part explain the improvement of the psoriasis during the development of lichen planus. Based on these immunohistochemical findings, we discuss a putative role for these lymphokines in the pathogenesis of the two diseases.