In an open and prospective study involving outpatient children with complicated urinary tract infections (UTI), we evaluated the therapeutic efficacy of ceftriaxone administered intramuscularly, once-daily - 50 to 70mg/kg, during 8 to 10 days. Initially, the selected patients exhibited at least two of the following clinical criteria: age below 6 months, any degree of toxicity, fever, strong suspicion or proved abnormalities of their urinary tracts and lumbar pain in children older than 4 years. Significant bacteriuria was demonstrated by urine culture in 40 patients (21 boys, 19 girls), whose ages ranged from 15 days to 6 years 9 months (median 3 months). The radiological studies revealed vesicoureteral reflux in 6 patients, urethral posterior valve in 1, and neurogenic bladder in 4. The main causative agents were Escherichia coli isolated in 30 patients, Klebsiella (4) and Proteus (4). The treatment was found to be effective in 38 patients (95%). There was failure of treatment in 1 patient and a symptomatic reinfection in another one. It was concluded that children with complicated UTI could be treated alternatively by once daily ceftriaxone.