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Low back (LBP) and chronic widespread musculoskeletal pain (CWP) both have a significant genetic component and are associated with increased body mass index (BMI). We examined whether LBP and CWP share common genetic factors, and to what extent this correlation is modified by the genetic factors influencing BMI. Genetic analysis of binary traits such as pain is not simple, particularly if their risk is associated with age or other quantitative traits. Implementing Falconer's polygenic threshold concept for dichotomous traits inheritance, we developed new software to examine the extent of the genetic influence on LBP and CWP under age and BMI dependence. The analysis was conducted on 3266 and 2256 UK female twins, assessed for LBP and CWP, respectively. Analysis of the liability scores with threshold to LBP and CWP established substantial contribution of genetic factors to their variation (h(2)0.60, p0.004-0.0003) and covariation (p=3.1E-08). Some 39% of the CWP and 70% of the LBP heritability estimates were attributable to genetic effects shared by both phenotypes, and 40% and 67% of the residual variation is caused by environmental factors simultaneously affecting both pain syndromes. However, contribution of BMI to variation/covariation of both pain phenotypes-although statistically highly significant (p∼10-7)-was not determinative.