Severe hypercalcemia (serum calcium, 4.37-4.84 nmol/L) was found in a 70-yr-old man who had a small cell carcinoma of the lung with multiple metastases. The plasma immunoreactive PTH concentration was markedly elevated, as measured in three different PTH assays [N-terminal PTH, 4,650 ng/L (normal, 230-630); midregion PTH, 13,850 ng/L (normal, 180-560); C-terminal PTH, 9,900 ng/L (normal, less than 1,300)], but at autopsy the parathyroid glands were histologically normal. The PTH concentration of a liver metastasis was 503.5 ng/g wet wt (normal liver, less than 4.2-5.9), and the PTH in the tumor extract eluted at nearly the same position as synthetic human PTH-(1-84) on gel filtration chromatography. Northern blot analysis revealed PTH mRNA in the tumor as a single band of 0.9 kilobase. These results indicate that the ectopic PTH production by the lung cancer was the cause of hypercalcemia in this patient.