This study was conducted to assess the hypothesis that mutations in the genes for sarcomeric proteins are the molecular cause for older-onset sporadic hypertrophic cardiomyopathy (HC) in Japanese patients. Molecular genetic approaches have demonstrated that familial HC is caused by mutations in genes encoding sarcomeric proteins. Recent studies have shown that sarcomeric gene mutations can also be a molecular cause of older-onset and/or sporadic HC. However, genetic studies to date have examined only a limited number of older Caucasian patients with HC. Clinical evaluations were performed in patients with HC onset after 40 years of age, and the sequence encoding the β-cardiac myosin heavy chain, cardiac troponin T, cardiac troponin I, cardiac myosin binding protein-C, myosin ventricular regulatory light chain, and myosin ventricular essential light chain genes was analyzed. When a putative mutation was identified, clinical evaluations and genetic studies were subsequently performed on all first-degree relatives. Forty-one patients with sporadic HC onset after 40 years of age (31 men, 10 women; mean age 63 ± 10 years at the time of study) were studied. Four novel missense mutations in the cardiac myosin binding protein-C gene (arginine to tryptophan at codon 160, glutamic acid to lysine at codon 334, glycine to arginine at codon 507, and threonine to methionine at codon 1,046) and a previously reported missense mutation in the β-cardiac myosin heavy chain gene (arginine to histidine at codon 663) were identified in 5 of the 41 patients. No family members carried these mutations or had clinical evidence of HC. In conclusion, mutations in the cardiac myosin binding protein-C are the most common cause of older-onset sporadic HC in Japan.