This paper describes a study of patients with cystitis treated with 1 gm/day of cinoxacin or four tablets/day of co-trimoxazole (trimethoprim, 80 mg, and sulfamethoxazole, 400 mg), both drugs given twice a day for 14 days. Of the 64 patients with cystitis, complete bacteriological data were available for 27 patients in the cinoxacin group and 23 patients in the co-trimoxazole group. In most instances, the infecting organism was Escherichia coli. Twenty-six (96%) patients who received cinoxacin and 22 (96%) patients who received co-trimoxazole had a satisfactory clinical response. Two patients on cinoxacin became reinfected with a new pathogen, and one had a recurrence of infection with the same pathogen; on patient on co-trimoxazole became reinfected with a new pathogen. Adverse reactions were reported by six (19%) of the 32 patients in the cinoxacin group, none of whom discontinued therapy, and by 18 (56%) of the 32 patients in the co-trimoxazole group, five of whom withdrew from the study. These differences between the groups were significant (P less than 0.05). It is concluded that cinoxacin is an effective, well-tolerated agent for use in cystitis caused by the common pathogens.