We are presenting two typical case histories of mucormycosis in patients with acute leukaemia which illustrate some of the important features of this infection in compromised hosts. In such patients, mucormycosis is mostly pulmonary or widely disseminated and often has a rapidly fatal course. A microbiological diagnosis from blood or sputum is difficult to establish. Mucormycosis or other fungal infections should be suspected in compromised patients with infections which do not respond to adequate antibiotic therapy.