This retrospective study aimed to evaluate the benefit of switching from calcineurin inhibitors (CnI) to sirolimus in posttransplant Kaposi's sarcoma (KS). Fourteen patients monitored in five French departments who had developed posttransplant KS were switched from CnI to sirolimus either abruptly (n=9) or progressively (n=5) with trough levels 5-12 ng/mL. Two patients had a complete remission, eight a partial response, and five no significant improvement of KS. The mean time to response was 3.9 months. After a mean follow-up of 16 months, 3 partial responders, with previous severe and refractory KS, suffered again from KS progression despite the lack of concomitant infectious or neoplastic event. These relapses occurred 5-9 months after switching. The tolerance of sirolimus has been excellent except for in one patient who developed severe interstitial pneumonitis. Sirolimus is usually useful in the management of posttransplant KS. It may be, however, ineffective or transiently effective in some patients with severe KS. Prospective studies with pharmacodynamic evaluation are important in order to better assess the duration of responses and the mechanisms of primary and acquired drug resistance.