The aim of this study was to assess the occurrence of non-sporeforming anaerobes in bronchoalveolar lavage in patients with pneumonia coexisting with lung cancer. Material consisted of 40 patients with lung cancer. Bronchoalveolar lavage was performed before administration of antimicrobial treatment and obtained material was cultivated in anaerobic and aerobic conditions. Quantitative assessment of anaerobic bacteria and their susceptibility to common antimicrobial agents were also performed. Peptostreptococcus, Prevotella, Fusobacterium and Bacteroides were the most common anaerobes. Among aerobic bacteria, the most frequent were G-negative bacilli. G-negative anaerobes were susceptible to most tested agents, whereas G-positive rods were resistant to metronidazole and tinidazole. This study demonstrates presence of non-sporeforming anaerobic bacteria in lower respiratory tract infections accompanying lung cancer.