Here, we present the case of a 74-year-old woman with clinically prominent hirsutism and a large adrenal mass where the clinical diagnosis was inconsistent with the histological appearance. Her hormonal evaluation showed normal androgens with slightly inadequate cortisol suppression upon dexamethasone test but normal 24-h urinary free cortisol. Abdominal computed tomography (CT) imaging showed a large inhomogeneous mass of the left adrenal and the FDG PET/CT scan revealed hypermetabolic foci in the right upper lobe of the lung, in hilum lymph nodes, and in the soft tissue in proximity to the right femoral neck. The cytological examination of a biopsy from the pulmonary lesion showed a highly malignant neoplasm of unknown origin, and left adrenalectomy, nephrectomy, and splenectomy were performed. The pathologists reported infiltration of the adrenal by an undifferentiated carcinoma of unknown origin. Due to the clinical suspicion of adrenocortical carcinoma (ACC), a mitotane therapy along with hydrocortisone substitution was directly initiated, and the patient also received 6 cycles of chemotherapy. However, the patient’s condition deteriorated quickly, and she died. Upon availability of steroidogenic factor-1 staining, the positivity of the resected tumor could be documented postmortem, confirming the diagnosis of an ACC. Taken together, this case underlines the difficulties in the differential diagnosis of aggressive adrenal masses.