Long term follow-up studies of children with congenital adrenal hyperplasia have documented less than desirable outcomes, including reduction in final adult height, obesity, virilism, and decreased fertility. We have proposed that children with the most severe forms of congenital adrenal hyperplasia would be better off if their adrenals were removed at an early age. We report here on our experience with prophylactic bilateral adrenalectomy in a 3-yr-old girl with a double null mutation of the CYP21 gene. The results of sodium balance studies, performed preoperatively on our patient and her unaffected fraternal twin sister, and hormonal data are presented as well. In contrast to her twin, who markedly increased her sodium retention in response to ACTH, our patient showed increased natriuresis, suggesting a deleterious effect of her adrenals on sodium homeostasis. Adrenalectomy was carried out at the time of necessary genital repair. No surgical or postsurgical complications were encountered.