Forty-nine girls between the ages of 2 and 18 years with a symptomatic urinary tract infection documented by two clean-catch urine cultures completed a double-blind study comparing the effectiveness of three days versus ten days of nitrofurantoin macrocrystal therapy. Localization of the infection to the lower urinary tract was presumed on the basis of clinical presentation. All patients had sterile urine on day two or three of therapy. In the ten-day group, two of 23 patients (8.7%) experienced a single relapse, and seven patients (30%) had 12 episodes of reinfection during a six-month follow-up. In the three-day group, two of 26 patients (7.7%) had a single relapse, and six patients (23%) had 12 episodes of reinfection. The rates of relapse and reinfection in the compared groups were not statistically significantly different (P greater than 0.05). Three days of treatment with nitrofurantoin macrocrystals is an effective regimen for symptomatic girls presumed to have uncomplicated lower urinary tract infections.