Pivmecillinam in a single bed-time dose of 5-10 mg/kg was given as long-term prophylaxis to 20 girls with recurrent bacteriuria for a total of 228 months. Symptomatic recurrences occurred in 4 girls, i.e. 1.8 infections/100 months as compared to 17/100 months during the year preceding the study. The frequency of positive urine cultures during prophylaxis was 11%. 10 of the 15 isolates were enterococci, resistant to mecillinam. In rectal swabs examined for the presence of aerobic gram-negative bacteria, enterobacteria were isolated in 78% with a low rate of resistance to mecillinam (11%). Thus pivmecillinam offered effective protection against recurrent urinary tract infections, and did not tend to select resistant enterobacteria in the bowel, but allowed resistant enterococci to cause a few symptomatic infections.