The development of biochemical and genetic screening tests for inherited endocrine diseases has dramatically changed our approach to surgical patients with endocrine tumors. Among more than 1800 patients with endocrine tumors and a possible inherited disease operated on between 1986 and 1997, there were 6.1% to 7.3% who were found to have a familial disease associated with familial medullary thyroid cancer, (MTC), multiple endocrine neoplasia type IIa (MEN-IIa), MEN-IIb, or MEN-I. Genetic testing for the RET proto-oncogene is therefore recommended for all patients with MTC, and testing for the MEN-I gene is recommended in patients with suspected MEN-I and in specific clinical subgroups with an increased probability of endocrine tumor heredity. Early treatment based on early diagnosis by genetic testing appears to improve survival and to decrease morbidity in these patients.