BACKGROUND: Steroid-refractory acute graft-versushost disease (aGVHD) is a serious complication after hematopoietic stem cell transplantation. The long-term outcome of the patients is poor. Various immunosuppressive agents have been proposed as the second-line therapy but none of them has turned out more effective than the others. Extracorporeal photopheresis (ECP) is a treatment option that does not predispose the patients to severe side effects of the immunosuppressive drugs. STUDY DESIGN AND METHODS: We analyzed the treatment results of ECP in 52 patients with steroidrefractory or steroid-dependent aGVHD. Eighty-one percent of the patients suffered from a severe, Grade III or IV, aGVHD. ECP was started alone as the second-line treatment in 23 patients and in combination with an immunosuppressive drug in 18 patients. Eleven patients received ECP as the third-line or later treatment. RESULTS: A total of 62% of the patients responded, with 48% achieving complete response. In the patients with complete or partial response, the probabilities of survival at 4 years were 54 and 17%, respectively. The outcome of nonresponders was poor. The 1-year overall survivals of the patients with ECP as the second-line treatment either alone or in combination with an immunosuppressive drug or as the third-line treatment were 51, 28, and 18%, respectively. In multivariate analysis, starting ECP no later than 10 days after the start of the first-line treatment correlated with a good response and a consequent survival benefit. CONCLUSION: Extracorporeal photopheresis is an effective and well-tolerated treatment that should be considered as a second-line treatment for aGVHD.