Upon entering the study, participants will have a baseline evaluation to measure the extent of GvHD. This assessment includes blood tests, eye and dental examinations, skin biopsy for patients with skin involvement, and CT scans and lung function tests to look for possible lung involvement. Biopsies of the lung, liver, mouth, or eye may be requested if needed to confirm GvHD in these tissues. The skin will be photographed before starting ECP treatment and once a month during the treatment period. Following baseline tests, participants will undergo treatment and evaluations as follows: