OBJECTIVES: To compare blood flow measurements of tumors assessed by perfusion computed tomography (pCT) and the clinical gold standard of 15O-labeled water positron emission tomography (15O-PET). METHODS: Blood flows were estimated by pCT (4-row multidetector, CT Perfusion 3.1) and 15O-PET (Posicam, first-pass model) in 14 patients with solid tumors, totaling 22 index tumors and 57 matched pairs of examinations. Blood flow estimates were compared using t test, Bland-Altman, and linear mixed regression analyses. RESULTS: There was no significant difference between the mean (SD) blood flow values measured by pCT and 15O-PET: 25.9 +/- 15.4 and 27.8 +/- 14.0 mL/min per 100 g, respectively. CONCLUSIONS: The demonstration of a good correlation between pCT and 15O-PET potentially enables the use of pCT, which is more widely available than 15O-PET, when tumor blood flow estimates are required, particularly in the context of clinical studies.