An 8 year old male castrated Russian Blue cat with polyuria, polydipsia, polyphagia, abdominal enlargement, unkempt and easily epilated hair coat and abdominal alopecia is described. As a first step diabetes mellitus was diagnosed. Further work-up by ultrasonography revealed severe bilateral enlargement of the adrenal glands. Hypercortisolism was suspected and therefore ACTH stimulation test and dexamethasone suppression test were performed. In all samples cortisol concentrations were below the detection limit of the assay used. Various precursor hormones were measured and high progesterone concentrations were found. Histologically, the adrenal masses were characterised as bilateral adrenal carcinomas of the adrenal cortex. The case report demonstrates that adrenal gland tumors are also capable to secrete sex hormones instead of cortisol. Clinical signs of hyperprogesteronism are identical to those of hypercortisolism.