Medullary thyroid carcinoma (MTC) occurs as a component of three well-described autosomal dominant familial cancer syndromes. Multiple endocrine neoplasia type 2A (MEN 2A) is characterized by MTC, pheochromocytomas, and parathyroid hyperplasia. Patients with the rarer multiple endocrine neoplasia type 2B (MEN 2B) syndrome develop MTC and pheochromocytomas, as well as mucosal neuromas, ganglioneuromatosis of the gastrointestinal tract, and a characteristic "marfanoid" habitus. Finally, MTC is transmitted in an autosomal dominant pattern in some families without associated pheochromocytomas or parathyroid hyperplasia (familial medullary thyroid carcinoma, MTC1(2). Sixty-one members of two well-characterized kindreds segregating MTC1 and 34 [corrected] members of six families segregating MEN2B were genotyped using a panel of RFLP probes from the pericentromeric region of chromosome 10 near a locus for MEN 2A. Statistically significant linkage was observed between the chromosome 10 centromere-specific marker D10Z1 and MTC1 (maximum pairwise lod score 5.88 with 0% recombination) and D10Z1 and MEN2B (maximum pairwise lod score 3.58 with 0% recombination). A maximum multipoint lod score of 4.08 was obtained for MEN2B at the position of D10Z1. In addition, 92 members of a previously unreported large MEN2A kindred were genotyped, and linkage to the pericentromeric region of chromosome 10 is reported (maximum pairwise lod score of 11.33 with 0% recombination between MEN2A and RBP3). These results demonstrate that both a locus for familial MTC and a locus for MEN 2B map to the pericentromeric region of chromosome 10, in the same region as a locus for MEN 2A. The finding that each of these three clinically distinct familial cancer syndromes maps to the same chromosomal region suggests that all are allelic mutations at the same locus or represent a cluster of genes involved in the regulation of neuroendocrine tissue development.