A method of measuring tumor blood flow by dynamic CT (DCT) was devised, and the reliability of data obtained by the new method was investigated. The subjects were 16 patients with superficial tumors who underwent measurement of tumor blood flow by both the new DCT-based method and 15O gas positron emission tomography (PET). The results showed that tumor blood flow as measured by the new DCT-based method was about 1.2 times greater than that measured by PET, and both data yielded a straight line passing almost through the origin (proportional relationships); the correlation coefficient was 0.836. On the basis of these studies, it was concluded that the newly devised DCT-based method yields reliable data in the measurement of tumor blood flow.