Using a double-blind controlled trial, we compared the clinical efficacy, safety and usefulness of cefotiam hexetil (CTM-HE) with those of cefaclor (CCL) in patients with bacterial pneumonia. Patients with obvious clinical signs and symptoms were given orally CTM-HE (at a daily dose of 600 mg) or CCL (at a daily dose of 1, 500 mg) for 14 days as a rule.We then assessed clinical and bacteriological efficacy, improvement in signs and symptoms, side effects, changes in laboratory findings and clinical usefulness.The results were as follows: 1) The clinical efficacy rate (excellent and good) was: in the committee's evaluation, 79.0%(64/81) for CTM-HE versus 83.3%(65/78) for CCL, and in the attending physician's evaluation, 77.0%(77/100) for CTM-HE versus 78.9%(75/95) for CCL.Thus, both evaluations generated similar results. 2) As to bacteriological effect, the eradication rate was 76.5%(26/34) for CTM-HE versus 83.9%(26/31) for CCL. 3) The safety evaluation showed that the two drugs were almost equivalent with regard to the incidence of side effects: 2.0%(2/102) for CTM-HE versus 3.0%(3/99) for CCL, and abnormal laboratory findings: 24.2%(23/95) for CTM-HE versus 18.7%(17/91) for CCL. With both drugs, gastrointestinal disorders and fever were found as accompanying symptoms, and most of the abnormal laboratory findings were eosinophilia and changes in liver function.No side effects were found particularly different from those reported with cephems.