The aim of this study was to identify mutations in the cardiac heavy-chain beta-myosin gene (MYH7b) in a group of Spanish patients with hypertrophic cardiomyopathy. The study included 36 families with at least one member who had hypertrophic cardiomyopathy. DNA from exons 3 to 24 of the MYH7b gene was sequenced. Two mutations were identified: Arg858Cys and Met515Val. They occurred in two families, one of which was of Moroccan origin. This corresponds to a MYH7b gene mutation frequency of less than 5%. In contrast to findings in other Caucasian populations, MYH7b gene mutation occurred infrequently in this group of Spanish families with hypertrophic cardiomyopathy.