Cutaneous T cell lymphoma (CTCL) is a clinically and immunologically defined neoplasm which encompasses epidermotropic (mycosis fungoides, Sézary syndrome) and nonepidermotropic variants. A natural evolution apparently occurs from the epidermotropic to the nonepidermotropic form. In this review, cellular properties of the neoplastic cells are correlated with specific clinical observations, and recent therapeutic advances are discussed. Advances in our understanding of the pathogenesis of CTCL, including preliminary evidence suggesting that keratinocytes may elaborate a hormonal substance capable of inducing T lymphocyte differentiation, are discussed.