Abstract: Extracorporeal photochemotherapy (ECP) has been progressively introduced into the treatment of both acute and chronic graft-versus-host disease (cGvHD) over the last decade. Nevertheless, its mechanisms of action, as well as the optimal treatment schedule, have not yet been defined. We retrospectively analyzed 25 patients with cGvHD unresponsive to conventional treatments who underwent ECP from 1997 until 2005. The impact of various factors (such as treated and infused nucleated cells, time from transplantation and cGvHD onset, and time from cGvHD and ECP treatment) on the probability of no response to ECP was therefore investigated. A positive response to ECP was achieved in 80% of the patients, after a median of 19 ECP treatments (with a range of 8–38). Eighteen out of the 20 patients responsive to the treatment maintained their response for a median of 30 months. We mainly focused on clinical response and yield composition. The analysis on mononuclear cell (MNC) dose suggested that an increase of MNC dose/kg b.w. (body weight) induced a decrease in the odds of treatment failure, and that, if the MNC dose infused was at least 100 × 106/kg b.w. per ECP treatment, a more positive and longer-lasting response was achieved. Moreover, the mean dose of treated and infused monocytes × 106/kg b.w./ECP did not account for a clear dose-related effect. These findings may eventually result in a more patient-tailored approach to ECP. Prospective multicenter trials should be designed to investigate the real impact of MNC dose on ECP responsiveness.