To determine the incidence of ranitidine-induced myelosuppression in the bone marrow transplant setting, we reviewed the records of all patients at our institution (223) who received ranitidine while undergoing bone marrow transplantation. We identified 37 cases in which a myelosuppressive episode was temporally associated with ranitidine use. In-depth analysis of the 37 cases showed 12 in which no alternative cause could be found and in which strong evidence for a ranitidine effect existed. Three of these cases are included herein as case reports. Based on this series, the overall incidence of ranitidine-induced myelosuppression in bone marrow transplant patients was approximately 5% (12/223).