A patient is described with a rapidly progressive Kaposi's sarcoma following immunosuppressive therapy with fludarabine monophosphate (FAMP) for chronic lymphocytic leukemia. The development of this tumor was accompanied by progressive thrombocytopenia, due to trapping of the platelets in the Kaposi sarcoma, without signs of consumptive coagulopathy or microangiopathic hemolytic anemia. This sequestration process might be caused by the abnormal structure of the tumoral vessels, leading to exposure of subendothelial structures like collagen, von Willebrand's factor, and tenascin to the vessel lumen. A severe immunosuppression due to the lymphocytotoxic effect of FAMP on several T-cell subpopulations was seen. This case confirms the importance of immunosuppression as a co-factor in the development of Kaposi's sarcoma also in HIV-negative patients, and adds Kaposi's sarcoma to the differential diagnosis of thrombocytopenia in immunosuppressed patients.