In 34 of 139 patients (63 men, 76 women, mean age 41.2 +/- 15.6 years) with medullary thyroid carcinoma (MTC) the disease was of the familial variety. 13 of the 34 cases were discovered in the preclinical stage by screening (calcitonin stimulation with pentagastrin). This group was on average younger (16.8 +/- 10.7 years) than the 21 clinically manifest patients with MTC (28.0 +/- 11.9 years) or the 105 patients with sporadic MTC (45.7 +/- 13.6 years). The patients diagnosed through the screening programme had a more favourable tumour stage (12 in stage I or II) and were thus more likely to be cured. Their survival rate was higher (100% at both 5 and 10 years) than the survival rate of the patients with clinically manifest tumour (94 and 87%, respectively). But the group with manifest familial MTC did not differ from those with sporadic occurrence as regards the tumour stage (predominantly advanced stages), limited chance of cure and survival rate (5 years: 94%, 10 years: 81%). These data show that screening of family members decisively improves the prognosis of MTC of the familial variety.