BACKGROUND Familial hypertrophic cardiomyopathy, an inherited primary cardiac abnormality characterized by ventricular hypertrophy, is the leading cause of sudden death in the young. Recent application of restriction fragment length polymorphism markers has provided provocative results, with localization to chromosome 18 (Japanese studies), 16 (Italian studies), 14 (US and French-Canadian studies), and two (National Institutes of Health studies) indicating genetic heterogeneity. Interpretation remains speculative until at least one of these loci is confirmed in unrelated pedigrees by independent investigators. METHODS AND RESULTS We studied eight unrelated families of varied ethnic origins across the United States. DNA from each individual was digested with restriction enzymes TaqI or BamHI and analyzed by Southern blots followed by hybridization with probes T cell receptor alpha (TCRA), myosin heavy chain beta, D14S25, and D14S26. Multipoint linkage analysis showed a maximum lod score of 4.3, placing the locus 10 cM from D14S26 between D14S26 and TCRA, with an odds ratio of 20,000:1 and 90% confidence limits of 12 cM proximal to D14S25 to 4 cM distal to TCRA. The probability of linkage to 14q1 was more than 99%. CONCLUSIONS These results indicate that the loci for familial hypertrophic cardiomyopathy in our families is primarily 14q1 but does not exclude other loci in a small proportion of the families. Thus, 14q1 appears to be the locus for familial hypertrophic cardiomyopathy in a significant proportion of the US population.