Acute graft-versus-host disease (aGVHD) is partly mediated through activated T cells, and these cells are known to express the high-affinity receptor for interleukin 2 (IL-2R). Denileukin diftitox is composed of human IL-2 and diphtheria toxin that is cytotoxic to activated lymphocytes expressing the high-affinity IL-2R. We describe the results of a phase II study of denileukin diftitox in 22 patients with steroid-resistant aGVHD. Twenty patients were treated at dose level 1 (4.5 microg/kg daily on days 1-5 and then weekly on study days 8, 15, 22, and 29), and 2 patients were treated at dose level 2 (9.0 microg/kg delivered on the same schedule). Dose level 2 was associated with grade 3/4 renal and hepatic toxicity and vascular leak syndrome, and no further patients were treated at this level. Dose level 1 was generally well tolerated. The response of aGVHD was assessed at study days 36 and 100. Nine patients (41%) responded, all with a complete response at study day 36, and 6 patients (27%) responded at study day 100 (4 complete responses and 2 partial responses). Denileukin diftitox has promising activity in steroid-resistant aGVHD, and further study is warranted.