Accumulating evidences suggest RET gene's involvement in development of the kidney in mice and humans. Although it is well known that RET mutation causes multiple endocrine neoplasia type 2A (MEN2A), thus far only 3 individuals have been reported to have MEN2A and renal agenesis/dysgenesis. We report a MEN2A family with RET mutation in which two asymptomatic carriers presented with unilateral renal agenesis. A 48-year-old woman underwent total thyroidectomy with regional lymph node dissection in our department for medullary thyroid carcinoma. She had earlier surgical treatment for a left adrenal pheochromocytoma at the age of 45. In the screening for MEN type 2 for her three sons, a CT scan for adrenal pheochromocytoma incidentally found unilateral renal agenesis in two of the sons, one of whom had suffered from Hirschsprung's disease (HSCR). They had contralateral kidneys exhibiting compensatory hypertrophy and normal renal function. Genetic analysis detected C618R RET mutation in the proband and her 3 sons, and no other mutations were found in RET as well as glial cell line-derived neurotrophic factor (GDNF). Our data lend support to the hypothesis that constitutive active RET mutation in MEN type 2 might partially impair RET function and thereby cause loss of function phenotype such as renal agenesis or HSCR.