A single-blind comparison was undertaken to evaluate 250 mg of cinoxacin and 100 mg of nitrofurantoin, both drugs being given four times a day, for the treatment of urinary tract infections. Complete data are available from 58 patients, most of whom were treated for 10 to 15 days. The clinical response was similar in both treatment groups, only one patient in the nitrofurantoin group having a recurrence of symptoms in the posttreatment follow-up period. In one patient in the cinoxacin group, the pathogen was not eradicated, and two patients who received nitrofurantoin had a reinfection with a new pathogen. Four patients stopped treatment early in the study because of side effects attributed to the trial drugs (three on cinoxacin, one on nitrofurantoin). Of the patients who completed the study, only six complained of side effects (two on cinoxacin, four on nitrofurantoin). The results of this study show that cinoxacin is a useful drug for the treatment of urinary tract infection due to susceptible organisms.