Previous findings show strong evidence for the role of retinoblastoma (Rb) in myoblast proliferation and differentiation. However, it is not known whether variation in the retinoblastoma gene (RB1) is responsible for normal variation in human muscle strength. Therefore, a linkage analysis for quantitative traits was performed on 329 young male siblings from 146 families with muscle strength, using a polymorphic marker in RB1 (D13S153 on 13q14.2). Trunk strength, a general strength indicator that requires activation of large muscle groups, was measured on a Cybex TEF isokinetic dynamometer. We found evidence for linkage between locus D13S153 at 13q14.2 and several measurements of trunk flexion with LOD scores between 1.62 and 2.78 (.002 < p < .0002). No evidence for linkage was found with trunk extension. This first exploration of the relationship between RB1 and human muscle strength through linkage analysis warrants efforts for further fine mapping of this region.