Four bone marrow transplant recipients consecutively occupying the same room on our Oncology-Hematology Special Care Unit (OHSCU) became colonized with Chaetomium species between January and April, 1987. These patients, aged 27 to 43 years, were immunocompromised as a result of intensive chemotherapy, and were consequently at increased risk for development of invasive fungal infection. At the time of Chaetomium colonization, all patients were febrile, two had transient new infiltrates on chest x-ray, and three were receiving amphotericin B therapy. Subsequent environmental cultures revealed Chaetomium contamination of the OHSCU air-handling system, including the HEPA (high-efficiency particulate air) filters in seven of the nine rooms comprising the unit. Because fungal colonization of HEPA filters used to create a “protective environment” for immunocompromised patients can occur and can serve as a source for patient infections, guidelines concerning proper surveillance of these HEPA filters should be established. We suggest that before a new patient enters a “protected” room, the clean side of the HEPA filter should be cultured. If fungi are recovered from that culture, we would recommend changing the filter.