An increasing incidence of invasive aspergillosis over the last decades is well documented in patients with haematological malignancies and is the most significant fungal infection in patients undergoing bone marrow transplantation and in aplastic anaemia. The diagnosis is difficult as clinical signs and symptoms usually are non-specific, but can be supported by frequent radiological examinations of the chest and sinuses and successive demonstrations of Aspergillus antigen in serum. The prognosis depends on the course of the underlying disease. A regeneration of the neutrophil granulocyte number is a condition for successful treatment. Early antifungal therapy is often necessary in neutropenic patients with fever and a lung infiltrate that does not remit following broad spectrum antibacterial treatment. Because of the risk of relapse after successful treatment these patients should receive prophylactic antifungal treatment during subsequent neutropenic episodes.