To determine how well tacrolimus (FK506) and cyclosporin A (CsA) are tolerated after HLA-identical blood stem cell transplantation, we performed a retrospective review of 87 adults transplanted consecutively who received FK506 (n = 40) or CsA (n = 47) in a nonrandomized fashion in combination with methylprednisolone for graft-versus-host disease (GVHD) prophylaxis and compared the incidences of complications potentially related to the immunosuppressive agents. Pre-transplant demographic characteristics, drug compliance and rates of acute GVHD were comparable for the two groups. Following first discharge, fewer patients in the FK506 group required antihypertensive therapy (32 vs 59%, P = 0.022), but more required insulin (34 vs 10%, P = 0.014). There was also a trend for more hyperkalemia and less moderate-to-severe venoocclusive disease in the FK506 group. However, nephrotoxicity, neurotoxicity, hemolytic-uremic syndrome, and cytomegaloviral or fungal infections through the first 100 days post-transplant did not differ significantly between the two groups. We conclude that for allogeneic blood stem cell transplant recipients, the incidence of complications related to FK506 and CsA in equally effective dose schedules in combination with methylprednisolone are similar with the exception of the risks of hypertension and hyperglycemia.