In 2005, the National Institutes of Health sponsored a Consensus Development Project on Criteria for Clinical Trials in chronic graft-versus-host (cGVHD) to achieve consensus about key elements of cGVHD research, including definitions for diagnosis, severity scoring, and response measures. To test these proposed definitions, a multicenter prospective cohort study of people with cGVHD is ongoing. This study will evaluate the performance of proposed prognostic factors, measures of disease activity, and surrogate endpoints for therapeutic response. Data are collected at 6-month intervals in a heterogeneous population of patients reflecting modern transplant techniques and posttransplantation clinical management (target enrollment 672 with cGVHD from 10 transplantation centers). This report describes the rationale, design, and methods of the cGVHD cohort study, and invites other investigators to collaborate with the Consortium to analyze data or specimens.