To date cardiac positron emission tomography (PET) studies have focussed on the measurement of myocardial blood flow, metabolism and receptors while left ventricular (LV) function and dimensions have been derived from other modalities. The main drawback of this approach is the difficulty of data co-registration, which limits clinical interpretation. The aim of this study was to evaluate whether it is possible to measure absolute cardiac volumes, and consequently LV function parameters such as ejection fraction, and wall motion with gated PET. Nineteen patients underwent a PET scan and planar radionuclide ventriculography (MUGA) within 9+/-9 days. A 9-min scan (16 gates/cardiac cycle) was acquired after inhalation of 3 MBq/ml of oxygen-15 labelled carbon monoxide at the rate of 500 ml/min over 4 min using a multislice PET camera. Noise reduction was performed on the gated image to enhance the definition of the ventricles before reslicing to the short-axis view. A threshold value was used to detect the edge of the LV at each gate. LV volumes at each gate were estimated by summing the volume of voxels within the LV boundary. PET measurements of LV volumes were as follows: LV end-diastolic volume ranged from 72 to 233 ml and LV end-systolic volume ranged from 24 to 203 ml. Phantom experiments supported the validity of this approach for estimating volumes. LV ejection fraction measured with MUGA was 38.4%+/-16.3% (range 15%-71%) and that measured with PET was 39.6%+/-17.7% (range 9%-72%) (P=NS). The LV ejection fraction measurements were highly correlated (r2=0.824). These results indicate that: (1) absolute end-diastolic and end-systolic volumes can be quantified using gated PET and (2) LV ejection fraction can be accurately measured by gated PET simultaneously with the other physiological PET parameters.