In the course of construction of a new bone marrow transplantation (BMT) unit, six of nineteen children undergoing BMT in the adjacent ward area died of invasive pulmonary aspergillosis (IPA). Ward air samples confirmed that heavy fungal spore contamination had occurred. The introduction of laminar air flow (LAF) isolation terminated the outbreak and no subsequent cases of IPA have been documented in patients nursed entirely in LAF. LAF has not reduced the rates of other complications associated with BMT.