Study objectives: To determine the type of infections occur-ring in hospitalized patients with lung cancer. Design: Prospective cohort study. Setting: Department of internal medicine in a cancer hospital. Patients: All patients with lung cancer who were hospitalized for any cause and who acquired infections at the time of admission or during the hospital stay between January 1997 and February 2001. Interventions: None. Results: Two hundred seventy-five patients with lung cancer bad 435 episodes of fever and/or microbiologically or otherwise documented infection. Two hundred eighteen patients (79.3%) presented with non-small cell lung carcinoma, while 49 patients (17.8%) had small cell lung cancer. The majority of the infections occurred in the tracheobronchial tree (56%). There were 38 episodes of bacteremia or fungemia, and the primary site of infection was identified in IS cases (47%). Microbiologically documented infections accounted for 61% of the infectious episodes, and included a total of 312 microorganisms. The most frequent pathogens were Gram-negative bacteria (64%), followed by Gram-positive bacteria (25%) and fungi (8%). The predominant Gram-negative bacteria were Haemophilus influenzae and Moraxella catarrhalis. Staphylococcus aureus, Streptococcus pneumoniae, coagulase-negative staphylococci, and Enterococcus faecalis essentially represented the Gram-positive bacteria. No multiresistant bacteria were observed. Bacteria were susceptible to most of the antibiotics classically administered for their treatment. Conclusions: The predominant site of infection in patients with lung cancer is the tracheobronchial tree, with S pneumoniae, S aureus, H influenzae, Escherichia coli, Pseudomonas aeruginosa, and M catarrhalis as the principal pathogens.