Forty-four isolates of Aspergillus fumigatus from patients with nosocomial aspergillosis and their hospital environment were collected and typed by immunoblot. Laser densitometric analysis of the profiles and homology studies within each case were performed which established relationships between patient and environmental types and showed that patients are often contaminated by several types of A. fumigatus, not all necessarily responsible for the mycosis. One of the cases appeared to be a typical clonal hospital-acquired infection. This typing technique seems to represent a powerful tool to determine the origin and mode of spread of nosocomial aspergillosis