Seven cases are presented of an exfoliative erythroderma with intense pruritus, edema, pigmentation, superficial lymphadenopathy, hepatomegaly, leonine facies, alopecia, dystrophic nails, and hyperkeratosis of the palms and soles. A cutaneous infiltrate of atypical mononuclear cells is present in this condition and is associated with an increased leukocyte count and with an abundance of these atypical cells in the peripheral blood. The cell is characterized by a large, convoluted nucleus with a narrow rim of cytoplasm frequently containing numerous vacuoles in a necklace-like arrangement around the nucleus. Within these vacuoles is an intensively PAS-positive, diastase-resistant neutral mucopolysaccharide. Patients with this disease remain in good general condition for a long time. Treatment with superficial irradiation, steroids, and cytotoxic agents yields only temporary relief. Death occurs approximately 5 years after the first symptoms have been noted.