This study is designed as a combined Phase II/III, randomized, open label, multicenter, prospective comparative study of sirolimus plus prednisone versus sirolimus/calcineurin-inhibitor plus prednisone for the treatment of chronic GVHD. Patients will be stratified by transplant center and will be randomized to an experimental arm of one of the two pre-specified experimental arms (sirolimus + prednisone or the comparator arm of sirolimus + calcineurin inhibitor + prednisone) in a 1:1 ratio.