This consensus document provides an update for pathologists and clinicians about the interpretation of biopsy results and use of this information in the management of hematopoietic cell transplantation patients. Optimal sampling and tissue preparation are discussed. Minimal criteria for the diagnosis of graft-versus-host disease (GVHD) are proposed, together with specific requirements for the diagnosis of chronic GVHD. Four final diagnostic categories (no GVHD, possible GVHD, consistent with GVHD, and definite GVHD) reflect the integration of histopathology with clinical, laboratory, and radiographic information. Finally, the Working Group developed a set of worksheets to facilitate communication of clinical information to the interpreting pathologist and to aid in clinicopathologic correlation studies. Forms are available at . The recommendations of the Working Group represent a consensus opinion supplemented by evaluation of available peer-reviewed literature. Consensus recommendations and suggested data-capture forms should be validated in prospective clinicopathologic studies.