ated single-photon emission computed tomography (SPECT) allows for the assessment of myocardial left ventricular ejection fraction and left ventricular volumes. There is conflicting data regarding the difference between automated and manual processing of gated myocardial SPECT images. The purpose of this retrospective “cross-sectional study is to compare the degree of variability between automated and manual processing of Quantitative Gated SPECT algorithms for assessing left ventricular volumes and ejection percent”. Study was carried out in the Nuclear Medicine department “at the Security Forces Hospital, Riyadh, Saudi Arabia, and comprised of 96 participants who “underwent” both stress and “rest gated” myocardial perfusion “imaging” (MPI) from February to May 2021. Data were analyzed “using SPSS for Windows (version 22; IBM Corp., Armonk, NY, USA). The mean of EF, ESV, and EDV on stress test using the automatic technique were 62.46 ± 14.62%, 45.00 ± 37.10 ml, and 107.01 ± 43.70 ml, respectively while using the manual technique were 57.21±14.80%, 44.35±35.93 ml, and 93.26±39.14 ml, respectively. The mean of EF, ESV, and EDV on rest test using the automatic technique were 62.85± 15.47%, 43.44± 36.35 ml, and 104.27±42.51 ml, respectively while using the manual technique were 57.74±15.33%, 43.30±36.42 ml, and 91.72±41.50 ml, respectively. For LVEF and EDV, the difference between automated and manual techniques is statistically significant (p<0.05). However, no “statistically significant difference” exists between automated and manual ESV techniques (p>0.05). Discrepancies were observed that exist by using the fully automated and manual technique for determining LVEF and Left ventricular volumes by gated MPI.