To pool results from studies of patients with hypertrophic cardiomyopathy (HCM) to elucidate important phenotypic differences among genotypes.Data published from November 1998 through November 2004 were gathered and compared from unrelated study population genotyping studies from the Mayo Clinic (Rochester, Minn), Harvard Medical School (Boston, Mass), France, Germany, Sweden, Finland, and Spain. Standard statistical analysis techniques were used to pool and compare data across genotypes with respect to frequency of mutations, age at diagnosis, and degree of hypertrophy (left ventricular wall thickness).The French study population harbored the highest frequency of mutations (61%), followed by the Mayo Clinic (38%), Harvard Medical School (36%), and Swedish (30%) study populations. For every study population, mutations in myosin binding protein C (MYBPC3) were the most common cause of HCM. Patients with a family history of HCM had mutations more frequently than those without. This pooled analysis revealed no statistically significant differences in left ventricular wall thickness or in mean age at diagnosis across all genotypes.Differentiation of sarcomeric genotypes, such as MYBPC3-HCM and MYH7-HCM, is not possible on the basis of currently reported phenotypic data. A myriad of genetic and/or environmental modifiers in addition to the primary disease-causing genetic substrate must play an important role in determining a patient's particular phenotype.