The gene encoding the beta-myosin heavy chain is one of the most frequently implicated in familial hypertrophic cardiomyopathy. Several mutations have been identified and some genotype-phenotype relationships have been assumed, particularly with regard to prognosis. Nevertheless, phenotypic expression is variable even in affected members of the same family carrying the same mutation. We identified the Ile263Thr mutation in several members of two unrelated Portuguese families. Penetrance, clinical behavior and prognosis were quite different between the two families, particularly concerning the occurrence of sudden death. Additional factors probably exist which account for the differences found. The complexity of hypertrophic cardiomyopathy makes it difficult to accurately determine genotype-phenotype relationships, and the screening and comparison of large affected families carrying the same mutation is warranted.