Legionella disease is among the leading causes of acute pulmonary infections. Nevertheless, the precise frequency remains difficult to assess because of its clinical aspects, the empirical treatment and the diagnostic requirements. According to the existing literature, the frequency of Legionella infections varies from 1 to 15% of documented pulmonary infections. In a survey carried out in 1990 in the Lyon area, the precise incidence of infections due to Legionella in daily medical practice has been estimated according to various types of practice. In general practice, only one case of Legionella infection was proved among 88 pulmonary infections. This weak incidence is also found in hospital practice: 10 cases among 398 hospitalized pulmonary infections in pneumology; none of 158 cases of severe pulmonary infections hospitalized in intensive care units. On the other hand, of 162 cases of hospitalized AIDS patients in a unit specialized in infections, two cases of Legionella infections were diagnosed. In a multidisciplinary 876 bed hospital, 16 cases of pulmonary infections due to Legionella were seen in 1990 by comparison with 32 cases in 1989 (chlorination of water since 1988). The data of Centre national de référence (Fleurette) report figures of 400 cases per year since 1985, without any change from one year to the other. These data are nevertheless underestimated. Global mortality is of 10%. No acquired resistance during the treatment is observed and infections caused by Legionella are consistently sensitive to macrolides, rifampicin and fluoroquinolones.(ABSTRACT TRUNCATED AT 250 WORDS)