In this paper we describe a patient with polycythemia vera (PV), who presented with hypercalcemia due to a parathyroid adenoma. In November 1999, the patient was admitted to our hospital with meteorism and constipation. Her physical examination revealed plethora and hepatosplenomegaly. Laboratory data revealed hyperparathyroidism in addition to PV: Rbc 8 X 10'(6)mm(3), Hct 63.7%, serum calcium 13.4 mg/dl, serum phosphorus 1.2 mg/dl, albumin 4.25 mg/dl, and alkaline phophatase activity 433 U/1. Intact Parathyroid Hormone level (iPTH) was 376 pg/ml (n.v.12-72 pg/ml). Twenty-four hour urinary calcium excretion was higher than normal (900 mg). A parathyroid adenoma was detected with Tc-99m sesta-MIBI scanning under the left lobe of the thyroid gland and an ultrasonographic examination of the neck also supported the diagnosis. The patient was recommended for surgery. The histopathological examination confirmed the diagnosis. Postoperatively, iPTH dropped to 53.4 pg/ml at the 15 th minute and to 33.5 pg/ml at the first hour. The calcium level was 7.5 mg/dl one hour after the operation. Five days later, Het was 40.8%. This case represents a rare association between PV and primary hyperparathyroidism, and may provide evidence for a causal link between PTH and polycythemia vera in our patient. In conclusion, this case indicates that the differential diagnosis of hypercalcemia and polycythemia vera should also include the possibility of a parathyroid tumor in addition to malignancy.