Deep fungal infections are an increasing problem in the treatment of acute leukemias and malignant lymphomas. Risk factors are known but unavoidable. Because of diagnostic difficulties most patients are treated empirically with intravenous amphotericin B. This drug's toxicity increases morbidity and mortality. An orally absorbable triazole derivative, itraconazole, may offer effective and safe prophylaxis against deep candidosis and aspergillosis in these patients. Such infections have been treated successfully with oral itraconazole even when resistant to intravenous amphotericin B. In retrospective comparative studies there are significantly less deep fungal infections in patients given itraconazole. The significance of the difference varies between studies. Pharmacokinetic data confirm therapeutic plasma levels of itraconazole but with wide variation within and between patients. The current large, multi-centre, randomised, double-blind, prospective trial of oral itraconazole versus placebo in the U.K. will test its prophylactic efficacy against deep fungal infections during treatment of haematological malignancies.