Objective By gene sequence ,biochemical and imaging testing, we introduce the diagnosis ,especially the predictive diagnosis of MEN2a. Methods Peripheral blood from 2 patients who were sisters presenting pheochromocytoma as the first manifestation of disease was tested for mutations of RET(exon 10 ,11, 12,13,14,15,16), VHL( exon 1 to 3), SDHD(exon 1 to 4), SDHB(exon 1 to 8) and SDHC(exon 1 to 6) by means of genomic polymerase chain reaction (PCR) amplification and DNA sequencing. The mutation affected RET proto-oncogene codon 634 and caused a cysteine to arginine substitution. Then we sequenced the other family members’ RET gene exon 11. Results We have detected point mutation in 5 MEN 2A patients in a MEN 2A family at codon 634 (TGC to CGC) in RET exon 11 which caused a cysteine to arginine substitution. Three of them were diagnosed as having bilateral adrenal pheochromocytomas by pathological examination and bilateral tumor in thyroid gland by ultrasound study. One patient underwent bilateral thyroidectomy and pathological examination showed medullary thyroid carcinoma. Conclusions The identification of a DNA alteration in the MEN2A gene will permit predictive diagnosis. Molecular testing of individuals at risk in these MEN 2A families is considered part of the standard management for at-risk family members.