Multiple endocrine neoplasia type 2A (MEN 2A) is an autosomal dominant disorder characterized by medullary thyroid carcinoma, pheochromocytoma and primary hyperparathyroidism. The first tumor is usually a medullary thyroid carcinoma. MEN 2A is caused by mutations in the RET proto-oncogene. The detection of mutations in the gene has important diagnostic and therapeutic impacts. Genetic testing of at-risk family members allows one to identify individuals carrying the mutant alleles with very high specificity and sensitivity. Subsequently, total thyroidectomy, recommended at 5 years of age, can be performed in a prophylactic attempt. The authors performed a molecular analysis to identify a mutation in a Thai woman with MEN 2A. She was found to be heterozygous for 1900T>C (C634R). The patient had two daughters who were not found to carry the mutation. The newly available genetic test for patients with MEN 2A in Thailand makes possible accurate DNA-based diagnosis of their at-risk family members before development of the disease, which has important therapeutic impacts for them.