The increasing incidence of human A. fumigatus infections particularly such of the respiratory tract has led to this study of the epidemiological situation. During the period between October 1968 and December 1977, A. fumigatus was isolated from clinical material in 425 cases. These findings which were established at the Mycology Unit, Robert Koch Institute, Federal Health Office, exhibited the following distribution by regions of the body or type of specimen: respiratory tract 277 (65.1(); venous blood 58 (13.6%); urine 3 (0.7%); stools 9 (2.1%) and others 78 (18.3%). Approximately two thirds of these findings were made during the period from October to March and about one third between April and September. The presence of A. fumigatus in the air both indoors and outdoors and also in the environment of patients suffering from aspergillosis was studied with the aid of the sedimentation-method. This method was chosen because the occurrence of A. fumigatus conidia in the air is of epidemiological interest. The number of isolations of A. fumigatus from outdoors samples was low, so the search for inhalative conidia concentrated upon sites near A. fumigatus habitats. These studies revealed that aspergillosis patients, clinical material sampled from them, decaying plant material (agriculture, horticulture), and used clothes and linen may form foci for the spread of A. fumigatus conidia. The control of aspergillosis in the hospital environment involves in particular a control of aspergillosis patients by means of culture and serology (preferably by ghe immunodiffusion test). Numerous recommendations are made on how to prevent A. fumigatus infections in hospitals, at the working site, and in the laboratory.