This report concerns control of adrenocortical carcinoma in a 4-yr-old boy by adjuvant mitotane therapy. He presented precocious puberty and was diagnosed with adrenocortical carcinoma. He underwent surgical resection, and adjuvant mitotane therapy was initiated, leading to a final dose of 5.0 g/day. Despite monitoring of the plasma mitotane level, encephalopathy developed 5 mo after initiation. Although he recovered from the encephalopathy, careful follow-up of his growth and development is necessary. On the other hand, he has been free of recurrence and metastases for 3 yr since discontinuation of mitotane. A high dose of mitotane is potentially effective as an adjuvant chemotherapy for adrenocortical carcinoma, although optimal and safe usage needs to be established for children.