A questionnaire comprised of a series of clinical vignettes illustrating potential difficulties in diagnosis, grading and treatment of chronic graft-versus-host disease (GVHD) was completed by 65 bone marrow transplant physicians from 51 transplant centers worldwide. The concordance index (CI) for responses to each question was calculated using the formula CI = 2p-1, where p was the percentage agreement among respondents. Possible responses ranged from 0 (no agreement) to 1.00 (perfect agreement). Responses indicated moderate (CI = 0.55), high (CI = 0.88) and moderate (CI = 0.54) agreement among transplanters for diagnosis, grading and treatment of chronic GVHD, respectively. Disagreement was observed in (1) diagnosis of uncommon manifestations of chronic GVHD; (2) in interpretation of symptoms which occur less than 2 months after transplant; (3) in interpretation of persistent stable symptoms; and (4) in deciding whether to treat chronic GVHD limited to skin. The results point to potential problem areas in designing and interpreting clinical trials to prevent or treat chronic GVHD and highlight the need for improved diagnostic criteria.