Familial multiple endocrine neoplasia type 2A (MEN 2A) is a cancer syndrome that is inherited as an autosomal dominant with high penetrance. Its clinical features are medullary carcinoma of the thyroid, pheochromocytomas, and hyperparathyroidism. A new polymorphic locus D10S97 (probe: KW6 delta SacI) detects a codominant EcoRI polymorphism that is tightly linked to the MEN2A locus. The peak lod score for linkage between D10S97 with MEN2A is 13.03 at theta = 0.00. The polymorphic locus D10S97 maps, by linkage analysis, into the previously defined interval between FNRB and RBP3 to which MEN2A has been assigned. We present physical mapping data showing that the probe pKW6 originates from 10p13 and that the polymorphic locus D10S97 in 10q11.2 is detected by cross-hybridization.