The therapeutic efficacy of cephacetrile (CEC) in bacterial pneumonia was evaluated in contrast with that of cefazolin (CEZ) by a double blind method. Both drugs were administered via intravenous route at a dose of 1 g twice daily for 14 days. 1) Of 81 patients, each 2 from both groups were eliminated from the study because of unknown results. In CEC group, 36 out of 38 obtained a slightly effective or better results (94.7% of effectiveness). In CEZ group, 31 out of 39 showed a similar result and there was no significant difference between the two groups. 2) In more detail, CEC achieved significantly better results in AaDo2 and cardiac insufficiency than CEZ, and this trend was also seen in dyspnea. 3) Regarding background factors, pretreatment severity was slightly in favor of CEC. However, so long as supplementary analysis is concerned, we could not find any relation between the pretreatment severity of symptom and drug efficacy or improvement of symptom. 4) Since there was a slight bias in the background factors, it is difficult to conclude that CEC is better than CEZ in terms of effectiveness. However, we consider CEC is superior to CEZ if compared in details. 5) Both drugs had the same incidence of side effect (6.25%, 3/48 in both groups). When clinical efficacy of CEC in bacterial pneumonia is evaluated together with the incidence of side effect, we may consider that CEC is an effective antibiotic agent equal to or better than CEZ.