We report a case of Kaposi's sarcoma in a patient who received a double kidney transplant in 2005. Immunosuppression was induced with rapamycin and antilymphocyte serum while maintenance therapy consisted of rapamycin, corticosteroids and mycophenolic acid. The patient developed delayed graft function but no rejection. In November 2006 and March 2007 two graft biopsies were taken because of a significant rise in serum creatinine; they revealed chronic allograft nephropathy and polyomavirus infection. Meanwhile a skin biopsy of the leg was performed to determine the nature of a discolored lesion. The morphohistological diagnosis was Kaposi's sarcoma. For this reason rapamycin was stopped and steroid treatment gradually reduced. Specific therapy with doxorubicin was started; radiological and endoscopic examination excluded disseminated disease while serological tests were positive for antibodies to HHV-8, a virus known to cause Kaposi's sarcoma. Unfortunately, withdrawal of antirejection therapy caused loss of the graft, so the patient had to start dialysis. In this report we stress the possible development of malignancy in transplanted patients who are given rapamycin. Rapamycin is known to be an antirejection drug and to have antineoplastic activity; the major risk of malignancy is probably related to immunosuppression rather than the type of drugs used to obtain it.