Abstract In a randomised controlled trial 45 children (47 episodes of urinary-tract infection) with radiologically normal urinary tracts were given either low-dose prophylactic co-trimoxazole or nitrofurantoin or no prophylaxis after each had been treated with a short course of co-trimoxazole. During prophylaxis none of the 25 children had a further infection whereas half of the 22 who received no prophylaxis had a further infection within a similar period. 8 (32%) of those children who received prophylaxis had a further infection within 1 yr of completing the course of prophylactic treatment compared with 13 (59%) of the no-prophylaxis group in the year following the initial course of co-trimoxazole. 24 children had had a previous urinary-tract infection. All 6 of these who did not receive prophylaxis had a further infection whereas only 8 of the 18 (44%) who had received prophylaxis had a further infection after the course of prophylactic treatment. 24 mos after the start of the study, 68% of the prophylactic group and 36% of those who received no prophylaxis were recurrence-free. Differences in symptomatology and timing of reinfections and possible contributory factors are discussed.