Denileukin diftitox (Ontak), a recombinant protein composed of human interleukin 2 (IL-2) fused to diphtheria toxin, has selective cytotoxicity against activated lymphocytes expressing the high-affinity IL-2 receptor. We conducted a phase 1 study of denileukin diftitox in 30 patients with steroid refractory acute graft-versus-host disease (GVHD). Seven patients received 9 microg/kg intravenously on days 1 and 15; 18 received 9 microg/kg intravenously on days 1, 3, 5, 15, 17, and 19; and 5 received 9 microg/kg intravenously on days 1 to 5 and 15 to 19. Hepatic transaminase elevation was the dose-limiting toxicity (DLT), and dose level 2 was the maximum tolerated dose (MTD). Overall, 71% of patients responded with complete resolution (12 of 24; 50%) or partial resolution (5 of 24; 21%) of GVHD. Eight of 24 patients (33%) are alive at 6.3 to 24.6 months (median, 7.2 months). Denileukin diftitox is tolerable and has promising activity in steroid-refractory acute GVHD.