Mycosis fungoides (MF) and Sézary syndrome (SS) are malignant non-Hodgkin's lymphomas, characterized by the proliferation of helper type T lymphocytes with a predilection for the skin. Because of the similarities in cytologic, histologic, cytogenetic, immunologic, and functional aspects of the malignant cells, as well as overlapping clinical features, these disorders are currently classified as cutaneous T cell lymphoma (CTCL). Though the etiology of these disorders remains obscure, environmental factors as well as viral infection have been implicated. In this study, seventy-six white patients with CTCL were typed for human leukocyte antigen (HLA)-A, -B, and -C to assess genetic susceptibility as determined by the major histocompatibility complex. An increase in the frequency of B8 and Bw35 was seen in SS patients but not in MF patients.