Sulfamethoxazole/trimethoprim (SMX/TMP) and nitrofurantoin are the most frequently used agents for prophylaxis to reduce the risk of recurrent urinary tract infections (UTIs) in children with vesicoureteral reflux (VUR). Nitrofurantoin, however, is not available in Japan and increasing resistance of organisms to SMX/TMP has recently raised doubts about its effectiveness as a prophylactic agent. This study was conducted to investigate whether antibiotic prophylaxis using low-dose cefaclor can effectively reduce the risk of recurrent UTIs. Thirty-nine children (31 male, 8 female) with primary VUR were enrolled. Ages varied from 0.5 to 111 months (mean 10.6 months). A prophylactic dose of 5-10 mg cefaclor per kg per day was given 1-3 times daily depending on the patient's age. Mean duration of prophylactic treatment was 15.5 months. Eleven children (ten male, one female) developed breakthrough UTIs during a total of 606 months treatment (or about one further infection in 55 months). Resistance to cefaclor was noted in three organisms: Enterococcus spp., Morganella spp., and Pseudomonas spp. Evidence of antibacterial activity was present in the morning urine samples from all of seven children tested. Cefaclor was well accepted and tolerated by all subjects. None withdrew from the study because of side effects. These results suggest that cefaclor can be an alternative choice for prophylactic treatment because of its safety, good compliance and low rates of resistant Escherichia coli.