Background: Patients with cutaneous T-cell lymphoma (CTCL; mycosis fungoides [MF] and Sézary syndrome [SS]) acquire immunodeficiency and opportunistic infections. Objective: We attempted to determine whether abnormalities of humoral immunoglobulin levels are present. Methods: A retrospective analysis of serum immunoglobulin levels in patients with CTCL at baseline evaluation at a cancer center was compared to levels in patients with leukemias and levels in healthy control subjects. Results: A total of 254 of 650 patients with CTCL evaluated between 1987 and 2001 had baseline quantitative immunoglobulin levels. Mean IgG, IgA, and IgM levels were similar among all MF/SS patients versus controls. The percentages of MF/SS patients with elevated levels of each immunoglobulin class were higher than percentages in healthy controls, and elevated IgA levels occurred among late versus early patients (P = .043). Conclusion: High immunoglobulin levels are more frequent in patients with MF and SS than in healthy controls, patients with chronic lymphocytic leukemia, and patients with hairy cell leukemia. High IgA levels are more frequent in late stage MF/SS. (J Am Acad Dermatol 2002;47:685-91.)