We carried out a well-controlled trial to evaluate the efficacy, safety, and usefulness of cefclidin (CFCL) in bacterial pneumonia, using ceftazime (CAZ) as the reference drug. Either drug was given intravenously in a dose of 1 g twice a day for 14 days. The following results were obtained: 1) The overall clinical efficacy rate determined by attending physicians was 86.9% (53/61) for CFCL and 90.3% (56/62) for CAZ. The efficacy rates judged by the Efficacy Assessment Committee (Study Committee) were 85.7% (54/63) for CFCL and 89.1% (57/64) for CAZ. There was no statistically significant difference in efficacy between the two drugs. 2) As for bacteriological responses, the eradication rate was 100% (28/28) in patients receiving CFCL and 96.7% (27/28) in patients receiving CAZ, with no statistically significant difference between them. 3) Adverse reactions were observed by attending physicians in 10/77 (13.0%) patients treated with CFCL and in 2/75 (2.7%) patients receiving CAZ (χ2-test, p<0.05). The incidence of adverse reactions was significantly higher for CFCL than for CAZ. The incidence of abnormal laboratory findings assessed by attending physicians was 16.4% (12/73) for CFCL and 21.1% (15/71) for CAZ. The Study Committee judged that adverse reactions occurred in 9/77 (11.7%) patients receiving CFCL and 2/75 (2.7%) patients receiving CAZ, with no statistically significant difference between the two drugs. In addition, the Study Committee judged that the incidence of abnormal laboratory findings was 27.4% (20/73) for CFCL and 31.0% (22/71) for CAZ, with no statistically significant difference between them. None of the symptoms or abnormal changes were severe. 4) The usefulness rate judged by attending physicians was 83.9% (52/62) for CFCL and 86.2% (56/65) for CAZ. The usefulness rate according to the Study Committee was 76.6% (49/64) for CFCL and 89.1% (57/64) for CAZ, with no significant difference between the two drugs. On the basis of these results, CFCL was concluded to be useful in the treatment of bacterial pneumonia, providing a high clinical and bacteriological response without severe adverse reactions.