Recently, the usefulness of circulating tumor DNA (ctDNA) analysis in various malignancies has been reported. However, reports on ctDNA analysis in adrenocortical carcinoma (ACC) are few. Therefore, this study aimed to examine the detectability of genetic mutations in ctDNA and the association between ctDNA allelic ratio and disease progression in a patient with post-operative recurrence of ACC. A 77-year-old woman presented with a 5.4 cm left adrenal mass, which was clinically diagnosed as subclinical cortisol-producing ACC on close examination. She underwent left adrenalectomy and was diagnosed with stage II (T2N0M0) ACC. Post-operatively, adjuvant chemotherapy with mitotane was commenced because of histologically high-grade ACC. However, 17 months post-operatively, she had a local recurrence at the left adrenalectomy site. FoundationOne