Thirty patients with presumed anaerobic pleuropulmonary infection were treated with clindamycin hydrochloride. Diagnoses of anaerobic infections were made primarily on clinical evidence, including history of aspiration, fetid sputum, and x-ray findings of pulmonary infiltrates, frequently with cavity formation. All patients were treated with clindamycin hydrochloride by mouth for 10 to 28 days in a dosage of 150 to 300 mg four times a day. Twenty-six patients showed a favorable response, although 6 also required closed thoracotomy with tube drainage of the pleural space. No serious side effects were encountered, although one patient developed transient neutropenia, and another showed elevation of hepatic enzyme levels. There were no cases of clinical superinfection, and only four patients acquired Gram-negative enteric pathogens in the sputum in low titers. Clindamycin given orally thus appears to be an effective initial antibiotic in the treatment of anaerobic pneumonias.