BACKGROUND We studied the need for sling surgery in patients who suffered from large cystoceles and masked stress urinary incontinence. METHODS Twenty patients who had large cystoceles but neither evidence nor history of stress incontinence were enrolled in this study. The cystocele was reduced using a reducing device. Masked urinary incontinence was identified by a 60 m pad weighing test and a stress test. The cystocele was reduced using a pessary ring in 14 patients, or a vaginal pack formed of two rolls of ordinary 28 x 28 cm gauze in six patients. Ten of 20 patients were diagnosed with masked stress urinary incontinence and were treated with anterior colporrhaphy and a suburethral sling procedure. The other 10 patients were continent after use of a cystocele reducing device and were treated with anterior colporrhaphy alone. Average follow-up periods of the patients with or without masked stress urinary incontinence were 51.2 months (range, 24.0-72.0 months) or 57.6 months (range, 27.0-70.0 months), respectively. RESULTS One of the 10 patients diagnosed with masked stress incontinence had mild stress urinary incontinence postoperatively. None of the 10 continent patients had stress incontinence after anterior colporrhaphy alone. CONCLUSIONS Reducing devices of protruding cystocele were clinically useful in the detection of masked stress incontinence. Sling surgery was effective to prevent emerging stress urinary incontinence for patients who suffered from cystocele and masked stress incontinence.