This retrospective study describes the outcome in 53 patients who had immunosuppressive treatment changed from cyclosporine (CSP) to tacrolimus for resistant acute GVHD (n = 23), hemolytic uremic syndrome (HUS) (n = 13) or CSP-associated neurotoxicity (n = 17). Tacrolimus was administered at doses of 0.03 mg/kg/day intravenously or 0.12 mg/kg/day orally in divided doses, as tolerated. Median time of conversion to tacrolimus after transplant was day 47. Nineteen patients had treatment changed to tacrolimus for resistant acute GVHD grades III or IV, with the median day of conversion being day 49 after transplant. Two of 20 evaluable patients had a complete resolution of GVHD after changing treatment to tacrolimus, with 18 showing no improvement. Eleven evaluable patients had therapy changed to tacrolimus for CSP-associated neurotoxicity at a median of 36 days after transplant. Eight patients had resolution of neurotoxicity and three had partial improvement. Eleven evaluable patients had therapy changed to tacrolimus for HUS at a median of 46 days after transplant. One patient had complete resolution of HUS and 10 showed no response. Side-effects related to tacrolimus included renal toxicity (34%), neurotoxicity (15%) and HUS (9%). Nine (17%) patients remain alive, including six patients who had therapy changed to tacrolimus for CSP-associated neurotoxicity. While often successful for dealing with neurotoxicity, only a rare patient improved after therapy was changed from CSP to tacrolimus for HUS or resistant acute GVHD.