BACKGROUND Recent identification of RET mutations in multiple endocrine neoplasia type 2A (MEN 2A) allows a DNA-based approach to diagnosis in lieu of calcitonin sampling. To prospectively evaluate the efficacy of mutational analysis, genetic screening was performed in 124 patients (53 male, 71 female; age, 1 month to 80 years) at risk for MEN 2A referred over 3 months. METHODS Analysis used genomic DNA and a polymerase chain reaction-based denaturing gradient gel electrophoresis strategy for mutation detection at RET codons 609, 611, 618, 620, and 634. Ninety-three of 124 patients were from established MEN 2A kindreds (group A), and screening replaced calcitonin testing. Twenty-one of 124 patients (group B) represented index cases of medullary thyroid carcinoma (MTC), and DNA analysis was performed to distinguish sporadic from hereditary disease. Ten patients (group C) had modest calcitonin elevations or had undergone thyroidectomy without confirming pathologic results, and testing was undertaken to clarify status. RESULTS Group A: RET mutations occurred in 29 (median age, 10 years) of 93 patients, 14 of whom underwent thyroidectomy. No false-positive results were observed. Group B: five (24%) of 21 patients with seemingly sporadic MTC had RET mutations at codons 618 (one), 620 (one), or 634 (three). Group C: Nine of 10 patients with alleged MEN 2A had genetically negative results. CONCLUSIONS Denaturing gradient gel electrophoresis reliably detects MEN 2A. Modest calcitonin elevations may lead to a false-positive diagnosis of MTC. DNA testing is the optimal approach to evaluating MEN 2A. Index cases of sporadic MTC should also undergo DNA analysis.