The efficacy of 3-day therapy with nalidixic acid in 16 children, and trimethoprim/sulphamethoxazole in 19 children, was studied prospectively in children with an acute infection of the lower urinary tract and compared with that of a conventional 10-day course with the same drugs. The immediate cure rate was 97% in the 3-day group and 90% in the 10-day group. During 3 months of follow-up there were altogether six re-infections in children given short-term treatment and six in the conventionally treated group. The results give further support for the suggestion that 3-day therapy is equivalent to 10-day treatment in uncomplicated urinary infections in children and that both nalidixic acid and trimethoprim/sulphamethoxazole are good alternatives in such an approach.