A developed urethral plug was evaluated for the treatment of women with genuine urinary stress incontinence. The plug consists of an oval meatal plate, a soft stalk and one or two spheres along the stalk with fixed distances between the meatal plate and the spheres. Inside the stalk is a removable semi-rigid guide pin to ease insertion. Forty women were randomly allocated to treatment with either the two-sphere or the one-sphere plug during period one (two weeks). In period two (two weeks) the patients used the other plug. They then continued with what they judged to be the better plug in period three (two months). Eighteen patients (45%) completed period three with the "preference" plug and 17 were subjectively and objectively continent or improved. Fourteen of these women preferred the two-sphere device. The plugs were equally effective in patients with mild or severe incontinence. Six women developed urinary tract infections and two of these had a plug in the bladder. The urethral plug is an effective treatment in a group of women with stress incontinence. Removal by hand is advisable in order to avoid retention of plugs in the bladder.