The multiple endocrine neoplasia type 2A (MEN 2A) gene has been mapped to the centromeric region of chromosome 10 by linkage analysis. We examined 36 medullary thyroid carcinomas (MTCs) (16 hereditary and 20 sporadic) and ten pheochromocytomas (eight hereditary and two sporadic) to detect loss of alleles on chromosome 10 using seven polymorphic DNA markers mapped to this chromosome. Of 20 informative cases, only one (5%) sporadic MTC showed loss of heterozygosity at the locus RBP3. Allele loss at the RBP3 locus was not found in pheochromocytomas from six heterozygotes. All tumors retained constitutional heterozygosity at six other loci on chromosome 10 (D10S17, D10S34, D10S24 on the short arm, D10S15 in the pericentromeric region, D10S20, and D10S4 on the long arm). Our findings suggest that the second hit for tumorigenesis in MEN 2A may not be loss of function of the normal allele at the homologous locus on the other copy of chromosome 10. Mutated ras oncogene was found only in one of 18 MTCs at the codon 61 of H-ras.