Since 1975, 10 families with the multiple endocrine neoplasia (MEN)-2A syndrome and five with the MEN-2B syndrome, making a total of 101 patients, have been identified in The Netherlands. Twenty-three of the MEN-2A patients died before the start of the screening program. The average age of the patients whose death was due to pheochromocytoma (n = 11) or medullary thyroid carcinoma (n = 12) was 34.9 and 49.2 years, respectively. Eighty-seven patients with the MEN-2A syndrome and eight with the MEN-2B syndrome underwent thyroidectomy for C-cell hyperplasia and/or medullary thyroid carcinoma. Eighteen patients had signs or symptoms caused by MEN-2A (group A), 60 were relatives of these patients who had been found to be affected at the first screening of the family (group B), and nine relatives had had negative screening results that later became positive (group C). Five patients had signs or symptoms due to MEN-2B (group A) and three were relatives of these patients who had been found to be affected at the initial screening (group B). To assess the effect of screening, we compared these groups with respect to the occurrence of metastatic medullary thyroid carcinoma at thyroidectomy and the results of the postoperative calcitonin tests. Among the MEN-2A families, 72 percent of group A, 33 percent of group B, and none of group C were found to have metastatic medullary thyroid carcinoma at surgery. In the MEN-2B families, all five patients in group A and one of the three patients in group B had metastatic disease. The "cure rates" in these three groups with MEN-2A, as determined by stimulated calcitonin assessment, were 11, 57, and 100 percent, respectively. One of the five patients with MEN-2B in group A and two of the three patients in group B showed normalization of the stimulated calcitonin value after surgery. From these results, it may be concluded that screening can lead to the detection of medullary thyroid carcinoma in an earlier stage, which in turn may permit curative treatment and improvement of both prognosis and life expectancy. The need for supervision of affected families by central registration to promote periodic examination and to guarantee the continuity of such screening is discussed.