Facioscapulohumeral muscular dystrophy (FSHD) is characterized by marked inter- and intrafamilial heterogeneity in its clinical expression. The contribution of genetic factors to this variability is not well characterized. We examined the relationship of phenotype to genotype in a clinically and genetically well-defined FSHD population. Quantitative isometric myometry (QMT) scores, normalized for age, gender, and height, were used to quantify disease severity. We found a significant (r = 0.92, p < 0.004) correlation between disease severity and the size of the 4q35-associated deletion. In addition, when relative disease severity of parent-offspring pairs was compared, the offspring were found to be significantly more severely affected (p = 0.011). This generational effect suggests the presence of anticipation in FSHD and raises the possibility of an underlying dynamic mutation.