The efficiency of treatment of acute urinary tract infections with sulfamethizole for 3 days, sulfamethizole for 10 days, and pivmecillinam for 3 days was compared in a randomized multicentre study comprising 264 girls aged 1-15 years. For ethical reasons children with complicated diseases were not included. In these treatment groups no significant growth after treatment was found in 81%, 77%, and 74%, respectively (NS). New bacteria after treatment were found less frequently after sulfamethizole for 3 days (4%) when compared to sulfamethizole for 10 days (14%) and pivmecillinam for 3 days (13%) (p = 0.048). After pivmecillinam treatment 75% of new bacteria were Streptococcus faecalis versus 25% after sulfamethizole for 3 days and 18% after sulfamethizole for 10 days (p = 0.016). In the subgroup with nephro-urological abnormalities no significant growth after treatment was found in 68% of the sulfamethizole 3-day treated group, 54% of the sulfamethizole 10-day treated group, and 67% of the pivmecillinam 3-day treated group (NS). All treatments resulted in a change in the bacterial sensitivity pattern when bacteria isolated 1-10 days after treatment was compared to those found before treatment. This was more pronounced after the 10-day treatment when compared to the 3-day treatment. The sensitivity patterns of the bacteria isolated from recurrences were similar to those seen before treatment. After treatment there was no difference in the actuarial percentage recurrence-free curves of the 3 treatment groups. Side effects were rare in the sulfamethizole treated groups, and seen more often in the pivmecillinam treated group. 3-day treatment with sulfamethizole or alternatively pivmecillinam is recommended as first choice for treatment of uncomplicated acute urinary tract infections in girls.