We report a unique case of a 44-year-old man with paraneoplastic hyperparathyroidism due to an oncocytic adrenocortical carcinoma (stage pT3N0R0M0, ENSAT 2 with a 4% Ki-67). Paraneoplastic hyperparathyroidism was associated with mild adrenocorticotropic hormone (ACTH)–independent hypercortisolism and increased estradiol secretion responsible for gynecomastia and hypogonadism. Biological investigations performed in blood samples from peripheral and adrenal veins revealed that the tumor secreted parathyroid hormone (PTH) and estradiol. Ectopic PTH secretion was confirmed by abnormally high expression of PTH mRNA and clusters of PTH immunoreactive cells in the tumor tissue. Double-immunochemistry studies and analysis of contiguous slides for the expression of PTH and steroidogenic markers (scavenger receptor class B type 1 [SRB1], 3β-hydroxysteroid dehydrogenase [3β-HSD], and aromatase) were performed. The results suggested the presence of two tumor cells subtypes with large cells with voluminous nuclei producing only PTH and that were distinct from steroid-producing cells.