Using metronidazole in oral dosages of 1.5 to 2 g daily, we treated 13 adults who had anaerobic pleuropulmonary infections, including 11 with lung abscess, one with necrotizing pneumonia, and one with thoracic empyema. Five patients (four with lung abscess and one with necrotizing pneumonia) were cured. The lung abscesses of 5 patients did not respond. For 3 patients (one with epigastric distress who refused metronidazole, one with undrained empyema, and one who died while receiving metronidazole), therapy could not be evaluated. Side effects included leukopenia (2 patients), leukopenia and neutropenia (one), neutropenia (one), dark urine (two), bitter taste (two), and epigastric distress (one). In light of our findings, metronidazole is not uniformly effective in the treatment of anaerobic pleuropulmonary infections.