If a participant's chronic GVHD improves at the end of the 16-week study duration, he or she may have the option of continuing the combination therapy of ECP and IL-2. Extended duration therapy is twice weekly ECP treatments plus daily IL-2 starting at the end of week 16. Participants may also have the option of continuing ECP treatments without IL-2 after the end of Week 16. If this is the case, participants will only be followed for one year from the start of therapy and will not have required study visits or tests.