1. All available evidence for and against the concept of ectopic hyperparathyroidism, including 307 case reports of tumor hypercalcemia, was collated. 2. Of 104 combined cases of tumors of the kidney, lung, liver, head, neck and esophagus, 91 (88%) were in men. 3. The parathyroid glands were examined in 170 of 307 cases and parathyroid hyperplasia or adenoma were described in 34 cases. This high frequency may reflect higher likelihood of reporting such association. 4. Analysis of the histological pattern of tumors associated with humoral hypercalcemia revealed a marked association with certain histological types in different organs, such as clear-cell carcinoma of the kidney and ovary, hepatocarcinoma and cholangiocarcinoma, pheochromocytoma, and squamous-cell carcinoma of the lung, head, neck, esophagus, and urogenital tract. This histological correlation is not compatible with the "random derepression" hypothesis. 5. The existence of tumor humoral hypercalcemia is well documented, as 61 of 74 operated patients sustained remission of hypercalcemia following tumor removal. 6. The evidence for ectopic PTH being produced by tumor is not well documented and is based on conflicting radioimmunoassay results. We have found no case in the literature which fulfilled unequivocally criteria of ectopic production of biologically active PTH. There has been a lack of studies of tumors for the presence of biologically active hypercalcemic factors because only relatively insensitive bioassays are available at present. More information is also required on microscopic bone changes in tumor hypercalcemia as x-ray studies alone are inadequate. 7. On the basis of the present evidence, causes and mechanisms of tumor hypercalcemia are likely to be multiple.