Ciprofloxacin treatment of urinary tract infections in 23 patients (16 with and seven without urorenal abnormalities) was compared with beta-lactam antibiotic therapy given orally to 10 comparable patients. The alteration of aerobic gram-negative fecal flora was monitored. Ciprofloxacin eliminated bacteria from the urine of all patients; beta-lactam antibiotics failed in two of 10 patients. There were no relapses one week following ciprofloxacin treatment; six of nine patients experienced a relapse following beta-lactam treatment (p less than 0.01). By four to six weeks, all patients infected with gram-positive cocci had a relapse. Six of 14 patients with urorenal abnormalities in whom cures had been achieved acquired new infections. Before treatment, the bowel flora of 80 percent of the patients was resistant to amoxicillin, 20 percent was resistant to cefixime, and none was resistant to ciprofloxacin. Penicillin and cefixime increased the incidence of resistant flora to 93 and 36 percent, respectively. Ciprofloxacin suppressed aerobic bowel flora, and no resistant strains persisted after treatment. Ciprofloxacin promises effective short-term oral treatment of complicated urinary tract infections with minimal risk of drug-resistant recurrences.