Aspergillus infections in four patients after cardiac surgery led to intensive search of the hospital environment for fungi and to review of contamination control practice. Aspergillus was grown from pigeon excreta on the outside windowsills and from moss growing on the hospital roof. It was recovered occasionally from the operating room and more often from the postoperative recovery room. Defects in the ventilating system of these areas were identified and corrected in order to reduce air contamination. Patient contamination must be related to the turbulent blood-air interface produced by cardiac suckers during operation, but still the manner of control and even the portal of entry of infection remain obscure and are, therefore, a cause for concern in future operations.