A C15O2 continuous inhalation and a steady-state theory using positron emission tomography (PET) have been used to quantitate regional cerebral blood flow. We extended this method for quantitative regional myocardial blood flow (RMBF) measurement. Forty-three patients, involving 4 normal volunteers, 16 patients with coronary heart disease (CHD), 20 patients with hypertrophic cardiomyopathy (HCM), and 3 patients with dilated cardiomyopathy (DCM), were studied. 6 min-scan was recorded in the steady-state condition during C15O2 continuous inhalation. A C15O inhalation scan was required to obtain the blood pool image. The subtracted image showed myocardial perfusion image. These images demonstrated homogeneous accumulation in normal volunteers. Reduced accumulation were detected in patients with myocardial infarction. Hypertrophied myocardium was demonstrated in patients with HCM. Diffuse heterogeneous accumulation was demonstrated in patients with DCM. Fourteen patients with HCM, whose left ventricular myocardial thicknesses were more than 20 mm, were selected for the RMBF measurement to minimize errors due to the partial volume effect. The arterial blood activity was measured by assigning a region of interest to the left atrial cavity on PET images. RMBF was calculated using the steady-state theory. Calculated flow values ranged from 21.2 to 152.6 ml/min/100 g. The good correlation was obtained between RMBF using 13NH3 first pass method and RMBF using gas inhalation method in six patients. These results indicate that C15O2 PET has the potential capability for the noninvasive quantitation of RMBF.