We report a case of lung infection by Pseudallescheria boydii, which appeared during therapeutic aplasia in a patient with acute leukaemia. The association of sudden chest pain and fever with negative blood cultures and haemoptysic expectoration first suggested a diagnosis of pulmonary aspergillosis, in particular because evidence of lung abscesses appeared very rapidly and large mycelial filaments were found on direct examination of the expectoration. Pseudallescheria boydii was identified by culture. Investigations in vitro of the effects of antimycotics showed that the micro-organism was resistant to amphotericin B and sensitive to imidazoles. The clinical and radiological outcome after treatment with ketoconazole was favourable.