Sezary cell leukemia (SCL) is a rare T cell neoplasia that has been suggested to be a variant of T-prolymphocytic leukemia (T-PLL). Both disorders have an aggressive clinical course, lymphocytosis with characteristic morphology, lymphadenopathy, hepatomegaly, characteristic cytogenetic abnormalities and mature T cell phenotypes. Skin lesions, however, are mainly found in T-PLL. We describe a patient with T-PLL/SCL, who atypically presented with severe seropositive polyarthritis and skin lesions, responding to treatment with human CD52 antibody, CAMPATH-1H and pentostatin. Meningeal leukemia and an assumed myocardial infiltration subsequently developed. Polyarthritis is common in T large granular lymphocyte leukemia and adult T cell lymphoma-leukemia, but both entities could be ruled out in the present case. In rheumatoid arthritis, an expansion of CD4+ and/or CD8+ T lymphocytes is well documented and this phenomenon is believed to be of pathogenetic importance. We speculate that the T cell clone in the present case had special homing properties or cytokine effects resulting in synovitis.