The autosomal dominant myopathy facioscapulohumeral muscular dystrophy (FSHD1, OMIM 158900) is caused by contraction of the D4Z4 repeat array on 4qter. We show that this contraction causes marked hypomethylation of the contracted D4Z4 allele in individuals with FSHD1. Individuals with phenotypic FSHD1, who are clinically identical to FSHD1 but have an unaltered D4Z4, also have hypomethylation of D4Z4. These results strongly suggest that hypomethylation of D4Z4 is a key event in the cascade of epigenetic events causing FSHD1.