Using metronidazole in oral dosages of 1.5-2 g daily, 13 adults who had anaerobic pleuropulmonary infections, including 11 with lung abscess, 1 with necrotizing pneumonia and 1 with thoracic empyema were treated. Five patients (4 with lung abscess and 1 with necrotizing pneumonia) were cured. The lung abscesses of 5 patients did not respond. For 3 patients (1 with epigastric distress who refused metronidazole, 1 with undrained empyema and 1 who died while receiving metronidazole), therapy could not be evaluated. Side effects included leukopenia (2 patients), leukopenia and neutropenia (1), neutropenia (1), dark urine (2), bitter taste (2) and epigastric distress (1). In light of the findings, metronidazole is not uniformly effective in the treatment of anaerobic pleuropulmonary infections.