In an open, prospective, multicenter trial the clinical efficacy of imipenem/cilastatin sodium (IPM/CS) for the treatment of 14 cases with aspiration pneumonia was investigated. The mean age was 75.4 years old. Diseases of central nervous system were present in 11 cases, cardiovascular diseases, pulmonary diseases and diabetes mellitus in 2 cases each respectively. Seven cases were community-acquired and another seven were hospital-acquired. Six cases were moderate and 8 cases were severe. Causative organisms were determined in 9 cases (64.3%), multiple causative organisms were isolated in 3 cases. Isolated organisms were Staphylococcus aureus (4), Pseudomonas aeruginosa (3), Klebsiella pneumoniae (3), Escherichia coli (1), Acinetobacter calcoaceticus (1). Detection of anaerobes was not attempted. Clinical effects of IPM/CS were excellent in 3, good in 8, fair in 2, poor in 1, the efficacy rate was thus 78.6%. P. aeruginosa was isolated from 2 out of 3 cases in which therapy with IPM/CS failed. Monotherapy with IPM/CS appears to be highly effective for cases of aspiration pneumonia, but the disease due to IPM-resistant P. aeruginosa is an exception.