OBJECTIVE: Many factors have been suggested to contribute to the development of urinary stress incontinence in women. One of the suggested factors has been attributed to a genetic defect in the connective tissue in such patients. The purpose of this study was to evaluate the prevalence of urinary stress incontinence among relatives of patients with urinary stress incontinence. STUDY DESIGN: The prevalence of urinary stress incontinence among first-degree relatives was investigated in 259 females with urinary stress incontinence. A control group comprising 165 women without micturition disorders was randomly selected from our gynecologic outpatient clinic. The two groups were matched according to age, parity, maximal birth weight, and percentage of first-degree relatives on whom information was obtained. RESULTS: Data from 780 first-degree relatives in the study group and 474 in the control group were evaluated. We found that there was an overall prevalence of 20.3% (178/780) first-degree relatives in the study group compared with 7.8% (37/474) in the relatives of the control group (p < 005). Among the mothers we found the prevalence of urinary stress incontinence to be 34.9% (71/203) in the study group compared with 12.7% (19/149) of mothers in the control group (p < 0.005). Among the sisters in the study group the prevalence of urinary stress incontinence was 19.9% (73/367) compared with 6.8% (15/220) of sisters in the control group (p < 0.005). Among the daughters we found the prevalence in the study group (6.7%) to be twice as high as in the control group (2.9%), but the difference did not reach statistical significance. CONCLUSIONS: The results of this study indicated a threefold prevalence of urinary stress incontinence among first-degree relatives of female patients with urinary stress incontinence. This finding may support the theory of a genetic factor in the cause of urinary stress incontinence. (AM J OBSTET GYNECOL 1996;174:617-9.)