This was a phase II, randomised, open-label, multi-centre study to assess the safety, tolerability, and efficacy of sitafloxacin (DU-6859a, 400 mg once daily) compared with imipenem (imipenem/cilastatin, 500 mg three times daily) in the treatment of hospitalised patients with pneumonia. Patients (n=69) were entered into the study in the intent-to-treat group, 35 in the sitafloxacin and 34 in the imipenem group. Patients (n=65) were included in the clinically evaluable population and 42 in the bacteriologically evaluable population. Baseline demographic data and clinical characteristics were similar for both treatment groups and across all patient populations. The incidence, severity and type of adverse events were similar in both treatment groups. The frequency of adverse events, which were considered to be related to the study of drugs was low and generally similar between the two groups. Mild transient increases in alanine aminotransferase and alkaline phosphatase occurred in the sitafloxacin treatment group, but there were no apparent trends in the other serum enzyme levels. The clinical response at the first and second follow-up assessments indicated that 94-97% of patients in the clinically evaluable population and 91% of patients in the intent-to-treat population were classified as cured in both treatment groups. The bacteriological response was classified as satisfactory for all patients (100%) in the bacteriologically evaluable population in the imipenem treatment group and satisfactory for 90 and 95% of cases at the first and second follow-up assessments in the bacteriologically evaluable population in the sitafloxacin treatment group, respectively. In conclusion, for the treatment of pneumonia, sitafloxacin was considered as safe and as tolerable as imipenem and preliminary data from this study suggest that it may have similar efficacy.