The patients were 117 children (aged 4 months to 14 years) with uncomplicated urinary tract infections caused by co-trimoxazole-sensitive Escherichia coli. The patients were randomly assigned to receive treatment with co-trimoxazole for 3 days (n = 58) or 7 days (n = 59). Urine was analyzed for bacteria before and immediately after treatment and again at 1 and 2 months. After 3 days' treatment, infection persisted in 14 of 31 patients with P-fimbriated strains of E coli and in 1 of 27 patients with non-P-fimbriated strains. After 7 days' treatment, infection persisted in 2 of 40 patients with fimbriated strains and in none of the 19 patients with nonfimbriated strains. One or 2 months after treatment, 3 days' treatment was rated successful in 26 of 27 patients with nonfimbriated strains and in none of the patients with fimbriated strains. Seven days' treatment was rated successful in all patients with nonfimbriated strains and in 32 of 40 patients with fimbriated strains. The results indicate that the length of treatment of urinary tract infections in children should be adjusted according to the presence of bacterial P-fimbriae in addition to the patients' clinical condition.