In an evaluation of 213 patients from 15 kindreds with familial medullary thyroid carcinoma (MTC), we detected 41 subjects from two kindreds (L and O) who had MTC but no extra-thyroidal manifestations (hyperparathyroidism, phaeochromocytomas or mucosal neuromas) of multiple endocrine neoplasia (MEN) type IIa or IIb. In screening 178 members of the L and O kindreds, we found no evidence that any of them had died from MTC. To assess whether the malignancy was relatively indolent in these families, 20 selected subjects from the two kindreds were compared with 33 MEN IIa subjects. Both groups had clinically occult disease which was diagnosed biochemically by documenting elevated plasma calcitonin (CT) levels following stimulation with intravenous calcium and pentagastrin. There were no differences in the peak stimulated plasma CT levels at the time of diagnosis (1055 +/- 236 pg/ml versus 1096 +/- 191 pg/ml) or the incidence of regional lymph node metastases (0/20 versus 1/33) in the two groups. The mean age at diagnosis, however, was significantly higher in patients of the L and O kindreds than in patients with MEN IIa (43.1 +/- 3.4 years versus 21.1 +/- 2.2 years; P less than 0.001) indicating that in the two kindreds the MTC either developed at a later age or grew more slowly. This study demonstrates that MTC may occur in a familial pattern distinct from its presentation as MEN IIa or MEN IIb. In this setting it appears to be the least aggressive form of MTC yet described.