The frequent occurrence of Legionnaires' disease outbreaks in many countries, including Italy, highlights the importance of epidemiological investigations on this disease. For this purpose, an active surveillance programme of pneumonia patients was carried out in a large hospital in Apulia (Italy) from March 2001 to December 2003. A total of 612 patients were tested for Legionella pneumophila infection by urinary antigen detection and evaluation of antibodies titre. Culture of respiratory specimens was performed on 117 patients. Thirty-seven patients (6%) tested positive for legionellosis: 10 resulted nosocomial cases and 27 as community-acquired. Only 9 cases di Legionella pneumophila infection were reported from Apulia to the routine surveillance system between 1996 and 2000. The results of our study denote the level of underestimation of this disease in Apulia.