Mitotane is used for treatment of advanced adrenocortical carcinoma. It is administered when the carcinoma is unresectable, metastasized, or at high-risk of recurrence after resection. In addition, mitotane is considered to have direct adrenolytic effects. Because of its narrow therapeutic-toxic range, therapeutic drug monitoring (TDM) is warranted. In 2020, a left-sided adrenal gland tumor was found (5.8 cm) in a 38-year-old man. Considering the size of this lesion and inability to exclude an adrenocortical carcinoma on imaging, a laparoscopic adrenalectomy was performed. Histopathologic examination determined presence of an adrenocortical carcinoma (pT2N0M0 ENSAT stadium II; ki67 10-15%). There was no evidence for residual or metastatic disease but given the high risk of recurrence, adjuvant therapy with mitotane was initiated. During TDM, a sudden and spuriously high level of mitotane was observed but without signs or symptoms of toxicity. After exploration, it was found that this high concentration was completely due to uncontrolled hypertriglyceridemia. After correction thereof, mitotane levels were again in the therapeutic range. This observation underscores the importance of TDM sampling in a fasting state with concurrent control of prevalent or incident dyslipidemia.