Eight patients with haematologic malignancies contracted fatal invasive aspergillosis during an outbreak. Five patients were neutropenic. Bronchofiberoscopic examination with microbiology specimen brush and bronchoalveolar lavage yielded Aspergillus fumigatus in only 2/5 patients examined. The specific diagnosis reached during lifetime in 5 patients was based on a combination of invasive procedures (lung biopsy in 2, percutaneous lung puncture in 1), the presence of a lung abscess (3 patients), seroconversion (1 patient), and purulent maxillary sinusitis caused by A. fumigatus together with repeated abundant growth of A. fumigatus in the sputum (1 patient). Six patients received amphotericin B. The infection was temporarily controlled only in 2 bone marrow transplant recipients whose granulocyte counts recovered. In 3/8 patients the pneumonia was of polymicrobial aetiology, Mycobacterium tuberculosis (2 patients), Pneumocystis carinii (1 patient), and Legionella pneumophila (1 patient) being the other microbes involved. 3/4 bone marrow transplant recipients with aspergillosis had been transplanted for chronic myeloid leukaemia, supporting the previously reported association of bone marrow transplantation for chronic myeloid leukaemia and the risk of invasive aspergillosis. Improved diagnostic methods for earlier definitive diagnosis of invasive aspergillosis as well as more efficacious and less toxic antifungal agents are needed to allow early treatment.