Seven patients who, following thyroidectomy for medullary carcinoma of the thyroid (MCT) had elevated peripheral basal or stimulated plasma calcitonin (CT) levels but no other evidence of disease, were prospectively studied to determine if disease could be localized by selective venous catheterization with pentagastrin stimulation. Disease was correctly localized to a surgically resectable area of the neck in every case and removed with low morbidity rates. Postoperative CT levels decreased in six of seven patients studied, suggesting reduction in tumor burden. One patient had no detectable CT following repeat selective venous catheterization with pentagastrin stimulation. Preoperative and postoperative carcinoembryonic antigen levels changed in a similar manner to CT levels in four of five patients studied. All patients remain alive 6 months to 3 years following the initial operation.