A 74-year-old Japanese male had developed generalized erythroderma and rapid growth of multiple verrucous lesions over the entire surface of his face, trunk, and extremities three months before he was seen. Histologically seborrheic keratoses were revealed. Laboratory examinations showed peripheral leukocytosis with atypical lymphocytes and high levels of IgE and IgG. On the basis of these clinical and histopathologic findings, we diagnosed the patient as having Leser-Trélat sign associated with Sézary syndrome. The erythroderma subsided after administration of oral predonisone, and no new formations of seborrheic keratosis were observed. However, because of subsequent aggravation of the generalized erythroderma, we administered chemotherapy. Six months after the initial examination, lung cancer was found, and the patient subsequently died of respiratory and renal failure.