Aspiration pneumonia, necrotising pneumonia and primary lung abscess are complications arising from the aspiration of infectious material from the oral cavity or stomach. There is limited information on optimal antibacterial therapeutic regimens. Patients with pulmonary infection following aspiration (n = 95) were included in a prospective, open, randomised, comparative multicentre trial to compare the safety, clinical and bacteriological efficacy of ampicillin + sulbactam vs. clindamycin +/- cephalosporin. Treated patients (n = 70) received sequential antibiotic therapy with either ampicillin + sulbactam (n = 37) or clindamycin (n = 33), with or without a second- or third-generation cephalosporin, administered until the complete resolution of clinical and radiological abnormalities. Definite or presumptive pathogens were isolated from 58 patients. Mean duration of therapy was 22.7 days for ampicillin + sulbactam and 24.1 days for clindamycin. In patients treated with ampicillin + sulbactam, the clinical response was 73.0% at the end of therapy and 67.5% 7-14 days after therapy. For clindamycin, the rates were 66.7% and 63.5%, respectively. Bacteriological response was similar in both treatment arms. Nine patients died (12.9%), with a Simplified Acute Physiology Score of > 30 points being the only significant predictive factor for therapeutic failure. Ampicillin + sulbactam and clindamycin +/- cephalosporin were both well-tolerated and proved equally effective in the treatment of aspiration pneumonia and lung abscess.