MEN-2 is a syndrome which is characterized by association of medullary thyroid carcinoma with pheochromocytoma. This syndrome is transmitted in an autosomal dominant fashion with very high penetrance. Linkage analysis of the kindreds at high risk and comparison of leukocytes- and tumor-DNAs of the patients using restriction fragment length polymorphisms (RFLPs) will be useful to find out the locus of the gene which predisposes to the syndrome. The environment factors play little role in the tumorigenesis in MEN-2.