A large number of families with familial isolated hyperparathyroidism (FIHP) have been reported. We wanted to determine if some of these families represent early manifestations of full-blown syndromes such as multiple endocrine neoplasia type 1 (MEN-1), as early identification may alter surgical and medical management. Four small families with a family history of hyperparathyroidism without clear-cut MEN-1 features were screened for a MEN1 mutation. The 10 exons of the MEN1 gene were amplified and analyzed by single-strand conformation analysis (SSCA). Abnormal SSCA shifts were then sequenced using an automated sequencer. Two germline mutations were found: R527X and P277H. The former was detected in three members of a family consisting of two children and a mother. At the time of testing the youngest son was normocalcemic and clinically normal but subsequently developed hyperparathyroidism (HPT). Since the initial testing, the family has been confirmed to be a MEN-1 family as the mother has developed abdominal pain and an elevated serum pancreatic polypeptide and the younger brother an anterior pituitary tumor and recurrent HPT. The latter P277H mutation was identified in two of three members tested from another family. Manifestations of MEN-1 syndrome have also developed. The father now has developed diarrhea and elevated serum gastrin; and the daughter has developed recurrent HPT. Genetic screening of families who clinically have FIHP is important and may influence the type of medical and surgical treatment and follow-up, as some have MEN-1 syndrome. Long-term screening for MEN syndromes should be included in this set of patients. Positive screening may predict disease and allow early detection and appropriate treatment before initiation of symptoms.