We evaluated patients presenting with large and recurrent sterile serosal effusions following bone marrow transplants. From a review of the Minnesota BMT Database from 1974 to 1993, seven patients with unexplained multiple effusions involving two or more of the pleural, pericardial or peritoneal cavities were identified. Patients with veno-occlusive disease (VOD), infections, cardiac insufficiency, tumor relapse and GM-CSF toxicity were excluded. All had onset following engraftment and six occurred before day 100. Unexplained multiple effusions were observed in recipients of allogeneic transplants but not autologous transplants and were found only in patients with acute and/or chronic GVHD. Five of seven patients also had cytomegalovirus (CMV) disease. Multiple effusions appear to be part of the presentation of severe acute or chronic GVHD, often in association with CMV disease in patients who receive allogeneic donor marrow.