Mycosis fungoides is a low-grade lymphoproliferative disorder caused by CD4+ lymphocytes. It is the most common type of cutaneous T-cell lymphoma. Typically, neoplastic T cells localize to the skin and produce patches, plaques, tumors, or erythroderma. Diagnosis of early mycosis fungoides can be difficult due to the nonspecific nature of cutaneous and histologic findings. However, recent advances in the application of histologic criteria, coupled with molecular biology tools such as immunophenotyping and polymerase chain reaction, have improved diagnostic accuracy. Independent prognostic factors include the extent and nature of skin involvement, the presence of extracutaneous disease, blood involvement, age > or = 60 years, and lactate dehydrogenase elevation. Accordingly, patients with limited patches and/or plaques (stage IA or IIA) experience long-term survival comparable to that of matched controls. The median survival is 11 years for patients with extensive patch/plaque (stage IB or IIA), 3.2 years for those with cutaneous tumors (stage IIB), 4.6 years for those with erythroderma (stage III), 1.2 years for those with pathologic nodal involvement (stage IVA), and 0.9 years for those with visceral disease (stage IVB). Over time, mycosis fungoides may progress to Sézary syndrome or transform to large-cell histology.