Early diagnosis and surgical treatment of medullary carcinoma of the thyroid (MCT) in children is essential to decrease the likelihood of metastatic spread. To determine the optimal timing of screening and surgery, we reviewed all children younger than 16 years old who were undergoing surgical treatment. From 1970 to 1988, 33 children 6 months old to 15.9 years old (mean, 9 years old) with MCT or C-cell hyperplasia or both underwent total thyroidectomy. Follow-up was 4 months to 15 years after surgery (mean, 9 years). Six of 31 patients presented with a neck mass. Basal immunoreactive calcitonin level was increased in 3 of 7 patients with C-cell hyperplasia alone, in 10 of 18 patients with nonmetastatic MCT, and in all 5 patients with metastatic MCT. At the time of surgery, 7 patients had C-cell hyperplasia, 19 patients had MCT without metastasis, and 7 patients had MCT with metastases. All children with metastasis were more than 12 years old. Medullary carcinoma of the thyroid was seen as early as age 6 months in a child with multiple endocrine neoplasia type IIb (MEN-IIb) and age 3 years in a child with MEN-IIa. At follow-up, 25 patients were free of disease and 8 had had recurrent disease. Two of the 8 patients with MEN-IIb had died. All 6 children with a neck mass at presentation had recurrent disease. Of 27 patients diagnosed by screening, 25 had no evident disease and 2 had persistent disease. Thus, in children with MEN-II syndromes, screening studies are highly effective in detecting MCT. All children with MEN-IIb should be studied shortly after birth, and those with MEN-IIa should be studied by 1 year old.