Medullary thyroid cancer may be autosomal dominantly inherited. Calcitonin is a very sensitive tumour marker in medullary thyroid cancer. It is essential to measure calcitonin in first grade relatives of these patients, in order to expose familial incidents of subclinical disease. In 55 patients with medullary thyroid cancer and no history of familial disease, nine close relatives with elevated calcitonin were identified in four different families. Eight of these nine have been thyroidectomized. Five were found to have medullary thyroid cancer, one had definite C-cell-hyperplasia, and two had equivocal C-cell hyperplasia. The last two, had elevated basal serum calcitonin-levels were increased, but with no further increase after intravenous pentagastrin bolus injection. Thyroidectomy did not modify these results and DNA-analysis may be necessary to draw a conclusion about the hereditary situation of these patients.