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Background: The relative importance of genetic and environmental factors for the occurrence of lower urinary tract symptoms (LUTS) is poorly understood., Objective: To (1) estimate the prevalence of urinary incontinence (UI), overactive bladder (OAB), and other LUTS and (2) to assess the heritability of these symptoms., Design, Setting, and Participants: Cross-sectional survey of LUTS in a national population-based cohort of Swedish twins 20-46 yr of age (n=42 582) from the Swedish Twin Registry., Measurements: Prevalence rates were determined and heritability of LUTS (in female twins) was assessed using indicators of twin similarity., Results and Limitations: A total of 25 364 twins completed the questionnaire (response rate: 59.6%). LUTS were more common in women (UI: 7%; OAB: 9%; nocturia: 61%; micturition frequency: 18%) than in men (UI: 1%; OAB: 5%; nocturia: 40%; micturition frequency: 11%), and prevalence increased with age. The strongest genetic effects were observed for UI, frequency, and nocturia. The lowest estimate for genetic effects was observed for OAB where environmental effects dominated, and more specifically shared family environment accounted for a third or more of the total variation. For stress UI, a fifth of the total variation in susceptibility to the disorder could be attributed to shared environment. Nonshared environmental effects were seen in the range of 45-65% for the various LUTS. The prevalence of LUTS was low in the men, and there were too few male cases to compute measures of similarity or heritability estimates., Conclusions: This study provides robust evidence of a genetic influence for susceptibility to UI, frequency, and nocturia in women. In contrast, shared environmental factors seem more important for the predisposition to develop OAB, which may reflect familial patterns such as learning from parental behaviours., (Copyright © 2011 European Association of Urology. Published by Elsevier B.V. All rights reserved.)