Five cases of aspergillosis complicating Hodgkin's disease and leukaemia are reported. The organs involved were: lungs (all five cases), stomach (Case 3); brain and meninges (Case 4); heart, kidneys, spleen, thyroid, and liver (Case 2). Cultures of Aspergillus fumigatus were obtained from the post-mortem tissues of three patients. All the lesions in Case 2 were suppurative. The other four cases had non-suppurative lesions characterized by spreading coagulation necrosis with peripheral hyperaemia, exudation, and haemorrhage. Invasion and occlusion of blood vessels occurred frequently. The various factors that may be responsible for the initiation and progression of the fungal infection are discussed. The available evidence suggests that Aspergillus fumigatus can produce toxic metabolites which are able to cause tissue necrosis and vascular damage. In patients suffering from neoplastic conditions of the lympho-reticular system, especially if steroid hormones or radiomimetic drugs are given, spreading, necrotizing lesions can develop unchecked by antibody or cellular defences.