A comparative clinical study of cefaclor (CCL) with cephalexin (CEX) was carried out by randomized double-blind techniques in order to contemplate the clinical efficacy, side effects and usefulness in treatment of 243 patients with dental infections. In the CEX group, CEX was orally administered 4 times a day at a daily dosage of 1,000 mg for 3 to 5 days. In the CCL group, CCL was orally administered 3 times a day at a daily dosage of 750 mg for 3 to 5 days and 1 capsule of placebo was also given every evening in order to keep the blindness of the administration. Evaluable cases for efficacy of athe drugs were 213 consisting of 108 for CCL and 105 for CEX. There were no significant differences in background of the patients and severity of the disease between 2 treatment groups. Clinical effectiveness on the 3rd day was 89.7% in CCL group and 78.6% in CEX group, showing significant difference between 2 treatment groups. Clinical effectiveness on the final day of administration was 94.4% in CCL group and 92.4% in CEX group, showing no significant difference between 2 treatment groups. Side effects were found in 10.5% of 114 patients receiving CCL and in 4.5% of 112 patients with CEX, and there was no significant difference between 2 treatment groups. The side effects were mostly gastrointestinal origin. According to the judgement by physicians in charge, no significant difference was seen in clinical usefulness between the 2 drugs.