We carried out a prospective study involving 96 consecutive lung cancer patients at diagnosis, in order to determine through quantitative cultures of the bronchoalveolar lavage (BAL) fluid, the prevalence of pulmonary infections; we also evaluated the relationship between a patient's performance status, immunocompetence, lung cancer stage, histotype and the occurrence of respiratory infections. The patients (81 males, 15 females) had a mean age of 64 +/- 9 years. Of these, 62 were smokers, 30 were ex-smokers and four had never smoked. Sixty-seven patients had a prior history of chronic bronchitis. A total of 42 micro-organisms were cultured from the BAL fluids of 33 patients (34.3%). Fifty percent of these micro-organisms were gram-negative, 33.3% were gram-positive and the remaining 16.7% were other micro-organisms. The bacilli most often isolated were the Haemophilus species, accounting for 38.8% of all gram-negative bacilli. The most frequently isolated gram-positive pathogen was the Staphylococcus aureus. We have not found a significant relationship between the presence of a respiratory infection and the different cell types separately analyzed, nor with SCLC and NSCLC patient groups, nor with the stage of the disease. The performance status, the immunoregulatory ratio and the lymphocyte subsets were not significantly different in patients with or without a pulmonary infection. We think that the identification of a definite etiologic agent is of great importance for a rational anti-microbial treatment of pulmonary infections.