The efficacy and safety of a novel once-daily extended-release ciprofloxacin (ciprofloxacin ER) 500-mg dose were compared with those of an immediate-release ciprofloxacin (ciprofloxacin IR) 250-mg twice-daily dose, each administered orally for 3 days in the treatment of acute uncomplicated urinary tract infection (uUTI) in women. Adult female outpatients (mean age, 39 years) with clinical signs and symptoms of acute uUTI and a positive pretreatment urine culture (>= 10(5) CFU/ml) were enrolled in a multicenter, randomized, double-blind, noninferiority trial. Patients were assessed at a test-of-cure visit (4 to 11 days posttreatment) and a late-posttreatment visit (4 to 6 weeks posttreatment) for microbiological and clinical outcomes and safety. The primary efficacy endpoint and microbiological eradication rate at the test-of-cure visit in the ciprofloxacin ER group (254/272; 93.4%) were noninferior to those in the ciprofloxacin IR group (225/251; 89.6%) (95% confidence interval [CI] of difference, -0.99%, 8.59%). Clinical-cure rates at the test-of-cure visit were 85.7% (233/272) for ciprofloxacin ER and 86.1% (216/251) for ciprofloxacin IR (95% CI of difference, -6.37%, 5.57%). At the late-posttreatment visit, microbiological and clinical outcomes were similar for the two treatments and consistent with test-of-cure results. Both treatments were well tolerated, but the frequencies of nausea and diarrhea were lower in the ciprofloxacin ER group than in the ciprofloxacin IR group (nausea, ER, 0.6%; IR, 2.2%; P = 0.033; diarrhea, ER, 0.2%; IR, 1.4%; P = 0.037). Once-daily ciprofloxacin ER was safe, effective, and noninferior to twice-daily ciprofloxacin IR in the treatment of acute uUTI. Additionally, ciprofloxacin ER was associated with significantly reduced frequencies of nausea and diarrhea.