BACKGROUND: There are few comparisons of antibody induction therapy allowing early glucocorticoid withdrawal in renal-transplant recipients. The purpose of the present study was to compare induction therapy involving alemtuzumab with the most commonly used induction regimens in patient populations at either high immunologic risk or low immunologic risk. METHODS: In this prospective study, we randomly assigned patients to receive alemtuzumab or conventional induction therapy (basiliximab or rabbit antithymocyte globulin). Patients were stratified according to acute rejection risk, with a high risk defined by a repeat transplant, a peak or current value of panel-reactive antibodies of 20% or more, or black race. The 139 high-risk patients received alemtuzumab (one dose of 30 mg, in 70 patients) or rabbit antithymocyte globulin (a total of 6 mg per kilogram of body weight given over 4 days, in 69 patients). The 335 low-risk patients received alemtuzumab (one dose of 30 mg, in 164 patients) or basiliximab (a total of 40 mg over 4 days, in 171 patients). All patients received tacrolimus and mycophenolate mofetil and underwent a 5-day glucocorticoid taper in a regimen of early steroid withdrawal. The primary end point was biopsy-confirmed acute rejection at 6 months and 12 months. Patients were followed for 3 years for safety and efficacy end points. RESULTS: The rate of biopsy-confirmed acute rejection was significantly lower in the alemtuzumab group than in the conventional-therapy group at both 6 months (3% vs. 15%, P<0.001) and 12 months (5% vs. 17%, P<0.001). At 3 years, the rate of biopsyconfirmed acute rejection in low-risk patients was lower with alemtuzumab than with basiliximab (10% vs. 22%, P = 0.003), but among high-risk patients, no significant difference was seen between alemtuzumab and rabbit antithymocyte globulin (18% vs. 15%, P = 0.63). Adverse-event rates were similar among all four treatment groups. CONCLUSIONS: By the first year after transplantation, biopsy-confirmed acute rejection was less frequent with alemtuzumab than with conventional therapy. The apparent superiority of alemtuzumab with respect to early biopsy-confirmed acute rejection was restricted to patients at low risk for transplant rejection; among high-risk patients, alemtuzumab and rabbit antithymocyte globulin had similar efficacy. (Funded by Astellas Pharma Global Development; INTAC ClinicalTrials.gov number, NCT00113269.). Copyright © 2011 Massachusetts Medical Society.