We have identified a new pathogenic mechanism for an inherited muscular dystrophy in which functional haploinsufficiency of the extracellular matrix protein collagen VI causes Bethlem myopathy. The heterozygous COL6A1 mutation results in a single base deletion from the mRNA and a premature stop codon. The mutant mRNA is unstable, subject to nonsense-mediated mRNA decay, and is almost completely absent both from patient fibroblasts and skeletal muscle, resulting in haploinsufficiency of the α1(VI) subunit and reduced production of structurally normal collagen VI. This is the first example of a muscular dystrophy caused by haploinsufficiency of a structural protein or member of the dystrophin-glycoprotein complex, and identifies collagen VI as a critical contributor to cell-matrix adhesion in skeletal muscle.