A blind prospective study was undertaken to determine the use of calcitonin (CT) as a tumor marker. After final diagnosis, results revealed elevated plasma CT (greater than 150 pg/ml) in common cancers as follows: lung, 38%; colon, 24%; breast, 38%; pancreas, 42%; and gastric, 30%. Fifty-eight percent of oat cell carcinomas were associated with elevated plasma CT. CT immunoreactivity was detected in 14% of tumor extracts and was not detectable in normal tissue other than thyroid. Hypercalcemia was not the cause of hypercalcitonemia. Incubation studies of [125I]human CT in cancer plasma and tumor extracts demonstrated that measurements were not an artifact of label degradation. In a survey of control patients with nonneoplastic disease, elevated CT was noted in renal failure, acute gastrointestinal bleeding, and in some patients with chronic obstructive lung disease. In conclusion, plasma CT is elevated in a substantial proportion of common neoplasms and is useful as a tumor marker.