Thirty-eight patients with erythroderma of unknown etiology were diagnosed over a 15-year period, and represented 19% of all patients admitted to our department for erythroderma. The male:female ratio was 6.6:1, and the median disease duration was 2 years (range 1 to 23 years). Keratoderma of palms and/or soles was seen in 79%. Laboratory findings were normal, except for an increased IgE level in 69% of the patients studied. Lymph node histology showed dermatopathic lymphadenopathy. Bone marrow investigation results were normal in 48%, or showed eosinophilia (32%) or hyperplasia (20%). Initial skin biopsies showed nonspecific histology in most patients, but later biopsies revealed pleomorphic infiltration. During the observation period four patients progressed to mycosis fungoides and another nine patients were suspected of having mycosis. None developed Sézary's syndrome. Only one third of the patients went into complete remission; half of them died during the observation period. Patients with erythroderma of unknown etiology are predominantly men and seem to belong to a certain subgroup--herein called the red man syndrome.