The gain achieved in image quality by utilizing, in the image forming process, the time-of-flight information (TOF) of positron annihilation photons between their inception and detection was measured experimentally by means of a positron emission tomograph (PET)-Super PETT I. The measurements were carried out by imaging a 35 cm cylindrical uniform phantom containing different positron activity concentrations. The gain achieved through the incorporation of TOF information, defined as the ratio of variances in images reconstructed with and without TOF information, was found to be approximately 3 at the lowest activity concentration and 5-8 in the activity concentration range typically encountered in clinical studies especially in fast or dynamic studies. This increase in gain with activity was interpreted as resulting from the reduction of random coincidences when TOF information is used. Further image improvement is yielded by incorporating TOF information into the PET attenuation correction provided by the measurement of transmission of annihilation photons in the object imaged.