When single photon emission computer aided tomography (SPECT) is performed, planar projection views are taken at a series of stepping angles covering the entire arc around the patient. These projection views are identical to planar gamma camera images, except that they are generally taken with a shorter acquisition time. The projection views are reconstructed to create transaxial SPECT images via backprojection techniques. We attempted the following studies to show that opposing SPECT projection views could yield the same correct quantitative information as the planar gamma camera images. Planar and SPECT images were used from patients who had received 0.135 MBq (5 mCi) of In-111-methyl benzyl DTPA BrE- 3 monoclonal antibodies. An In-111 filled flat flood source was utilized to acquire transmission images on the planar gamma camera in order to generate an attenuation map of the patient in the anterior/posterior plane. A camera calibration factor was obtained using a source of known activity. The activity of the liver was determined from abdominal planar images using regions of interest (ROIs) drawn around the liver on opposing anterior and posterior views. Similarly, the activity of the liver was determined from the opposing SPECT projection images which showed the anterior and posterior views. The same attenuation map was used for the correction of both the planar images and SPECT projection views. A cylindrical plastic phantom containing spherical plastic balls was used to validate that this technique accurately measured the activity contained in selected ROIs. Planar and SPECT images were taken of the phantom with each ball containing 6.8 kBq (250 μCi) of Tc-99m. A Tc-99m filled flat flood source was utilized to acquire transmission images on the planar gamma camera, and a source of known activity was used to obtain a camera calibration. Using a similar method to that used on the liver images, the activity of the balls was determined from the planar images and the SPECT projection views. Liver activities calculated from SPECT projection views matched the activities calculated from the planar images within 20% error. The activities for each ball in the phantom, calculated from SPECT projection views, matched the activity calculated from the planar images within 5% error, and matched the known activity in each ball within 10% error. The data indicates that despite their short acquisition times, the SPECT projection views may be used to quantify the activity from ROIs, without a significant increase in error associated with activity measurement.