Improvement in chronic graft vs. host disease (cGvHD) following treatment with extracorporeal photopheresis (ECP) has been shown previously. However, the effect is often measured at only one point in time or as best response. Chronic GvHD activity fluctuates over time, so we retrospectively evaluated cGvHD responses in 54 patients with primarily moderate or severe cGvHD throughout the ECP treatment course and after stopping ECP. The dominant response was partial remission (PR) in 33 patients, no change (NC) in 10 patients, progressive disease (PD) in 10 patients and complete remission (CR) in one patient. Response rates and reduction in glucocorticoid dose reached a plateau after nine months. The main reason for stopping ECP was the absence of further improvement. Flares in cGvHD activity were seen in 36 patients. Additional treatment during ECP was administered to 29 patients. Failure free survival with response was achieved for 52% of patients at 6 months and 43% at 1 year. Our study confirms that ECP is a safe option for cGvHD therapy. The majority of the patients experience improvement and reduction in glucocorticoid dose but flares in cGvHD activity and the need for additional immunosuppression are seen frequently.