Background. Positron emission tomography (PET) with inhaled oxygen 15-labeled carbon monoxide (CO) is used as a marker of myocardial blood pool. Only a limited number of studies with small numbers of patients have reported on the assessment of left ventricular (LV) volumes by use of O-15-labeled CO. The aim of this study was to compare LV volumes and function as measured by routinely acquired blood pool images by use of gated O-15-labeled CO PET with the reference technique, cardiovascular magnetic resonance imaging (MRI). Methods and results. Thirty-four subjects with a varying degree of LV function were studied. LV end-diastolic volume (LVEDV), LV end-systolic volume (LVESV), and LV ejection fraction (LVEF) were determined by both MRI and gated PET by use of O-15-labeled CO. Volumes were comparable with respect to LVEDV (196 ± 83 and 192 ± 91 mL, respectively; P = not significant). LVESV, however, was slightly overestimated by PET (119 ± 85 and 136 ± 94 mL, respectively; P < .05), resulting in a significant underestimation of LVEF (44% ± 19% and 35% ± 18%, respectively; P < .05). Observed correlations for LVEDV, LVESV, and LVEF were 0.90, 0.96, and 0.86, respectively (all P< .01). Conclusions. Gated O-15-labeled CO PET measurements of LVEDV, LVESV, and LVEF show good correlation with MRI over a wide range of LV volumes during routinely acquired blood pool images. LVEF, however, may be underestimated compared with MRI.