Patients with acute leukaemia or undergoing allogenic bone marrow transplantation at University College London Hospital Trust were screened for the presence of aspergillosis by polymerase chain reaction (PCR). Aspergillus DNA, from whole blood samples, was amplified by nested PCR to detect a 135 bp fragment in the mitochondrial region of Aspergillus fumigatus or A. flavus (121 bp). One colony-forming unit (CFU) per 2 ml of blood or 1-10 fg DNA could be detected. Patients at risk of aspergillosis were classified as probable or possible based on the European Organization for Research and Treatment of Cancer definitions. Antifungal drugs given were recorded. In four of 17 patients studied, infection was not suspected and the PCR was negative. Four patients were considered to have possible aspergillosis infection and were PCR positive on at least one occasion. Of the three patients in the probable group, four of the nine samples tested PCR positive from one patient and in another patient only one of nine samples tested positive. The remaining six patients were not suspected of having fungal infection but each had one or two PCR-positive results. In summary, six of seven patients thought to have clinical evidence of infection were PCR positive on at least one occasion and treatment with antifungals may have reduced infection below detectable levels.