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Posttransplant hemolytic uremic syndrome (pHUS) is a rare but severe disorder that confers a poor prognosis on an allograft due to thrombotic microangiopathy. Immunosuppression with cyclosporine (CsA) is implicated as a significant risk factor for the development of pHUS. In early reports, it was hypothesized that immunosuppresion with FK506 (tacrolimus) would avoid the development of pHUS. However, this initially supposed beneficial effect remains controversial, because pHUS associated with tacrolimus therapy has been published in some later case reports. This article aims to further evaluate FK506 with respect to the development and resolution of pHUS.We describe the course of seven adult kidney graft recipients with pHUS, treated with FK506 either as initial immunosuppression for retransplantation or after discontinuation of CsA for resolution of pHUS. Work-up for pHUS was initiated when certain clinical features, such as hemolytic anemia, thrombocytopenia, and deterioration of graft function, were found. The diagnosis was confirmed by histologic examination of a renal allograft biopsy specimen (thrombotic microangiopathy). With the onset of pHUS, additional plasma exchange was performed in all patients.Two patients suffered from end-stage renal disease due to primary HUS and had a history of recurrent pHUS in previous renal transplants. In both patients, the attempt to regraft was only made because of the early optimistic reports using FK506. Despite initial FK506 therapy, both recipients developed pHUS again, leading to loss of graft function. Two additional kidney graft recipients with primary renal failure other than HUS also received FK506 as initial immunosuppression. One of them (loss of the first kidney graft due to CsA-induced pHUS) was successfully treated with FK506 for his second renal transplant. The other recipient, a patient in whom de novo pHUS had occurred in the first graft despite initial therapy with FK506, was treated with CsA for his second graft and again developed pHUS. The latter process, however, could be reversed by a switch to steroids and azathioprine. In all three patients regrafted for reasons other than pHUS, development of de novo pHUS was treated by CsA withdrawal and a switch to FK506; this approach was effective in two patients.Our results demonstrate that three of seven renal allograft recipients benefited from FK506 therapy for prevention or resolution of pHUS. Treatment or prophylaxis with FK506 can be considered advantageous in some patients with de novo pHUS, but FK506 fails to prevent recurrent pHUS in patients with primary HUS.