Patients with idiopathic erythroderma have often been regarded to have a pre-Sezary syndrome because some of them have developed a cutaneous T-cell lymphoma during follow-up. Sezary syndrome is a form of leukemia-lymphoma characterized clinically by erythroderma, pruritus, adenopathy, and circulating atypical cells with cerebriform nuclei. We describe a case of Sezary syndrome in a 40-year-old man, who suffered from idiopathic erythroderma for 3 years. We suggest that close and long-term follow-up should be performed on patients with idiopathic erythroderma. (Korean J Dermatol 1999;37(4) : 514-518)