C-protein is a major component of skeletal and cardiac muscle thick filaments. Mutations in the gene encoding cardiac C-protein [cardiac myosin binding protein-C (cMyBP-C)] are one of the principal causes of hypertrophic cardiomyopathy. cMyBP-C is a string of globular domains including eight immunoglobulin-like and three fibronectin-like domains termed C0-C10. It binds to myosin and titin, and probably to actin, and may have both a structural and a regulatory role in muscle function. To help to understand the pathology of the known mutations, we have solved the structure of the immunoglobulin-like C1 domain of MyBP-C by X-ray crystallography to a resolution of 1.55 A. Mutations associated with hypertrophic cardiomyopathy are clustered at one end towards the C-terminus, close to the important C1C2 linker, where they alter the structural integrity of this region and its interactions.