Multifocal alveolar hypoxaemic pneumonia caused by sporadic nosocomial infection led to acute respiratory failure and development of cavitations. Legionella pneumophilia type I was isolated. The patient was not in a state of immunosuppression other than that due to Cushing's disease recently treated surgically followed by corticosteroids. Specific clinical and radiological signs of Legionellosis (particularly the development of cavitation) and their mode of infection (community acquired or noscomial, sporadic or epidemic, immunocompetence or immunodepression) are discussed.