Purpose: We compared estimates of ejection fraction (EF) determined by gated Tl-201 perfusion SPECT (g-TI-SPECT) with those by gated blood pool (GBP) scan. Materials and Methods: Eighteen subjects underwent g-TI-SPECT and GBP scan. After reconstruction of g-TI-SPECT, we measured EF with Cedars software. The comparison of the EF with g-TI-SPECT and GHP scan was assessed by correlation analysis and Bland Altman plot. Results: The estimates of EF were significantly different (p<0.05) with g-TI-SPECT (40%±14%) and GBP scan (43%±14%). There was an excellent correlation of EF between e-TI-SPECT and GBP scan (r=0.94, p<0.001). The mean difference of EF between GRP scan and g-TI- SPECT was +3.2%, Ninety-five percent limits of agreement were +9,8%. EF between g-TI-SPECT and GBP scan were in poor agreement. Conclusion: The estimates of EF by g-TI-SPECT was well correlated with those by GBP scan. However, EF of g-TI-SPECT doesn't agree with EF of GBP scan. EF of g-TI-SPECT cant be used interchangeably with EF of GBP scan. (Korean J Nucl Med 2000;34:222-7)