Background GIK-²⁰¹Tl imaging reportedly improves the detection of viable myocardium, so the present study evaluated whether it can detect myocardial viability after acute myocardial infarction (AMI). Methods and Results Resting ²⁰¹Tl and ^99mTc-pyrophosphate (PYP) dual single photon emission computed tomography (SPECT) and ²⁰¹Tl SPECT after ²⁰¹Tl with GIK (10% glucose, insulin 5 U, and KCl 10 mmol) infusion (GIK-²⁰¹Tl) were performed in 25 AMI patients within 10 days of admission. GIK-²⁰¹Tl SPECT images were obtained immediately and 4 h after infusion. Left ventriculography (LVG) was performed within 3 weeks and at 6 months when follow-up ²⁰¹Tl SPECT was also performed. From 20 SPECT segments, both the summed defect score (RDS) and the number of defect segments (ES) were calculated. The infarcted area was defined as ^99mTc-PYP uptake segments. Wall motion was estimated in 7 LVG segments. The ES of R-²⁰¹Tl (5.5±2.8), immediate GIK-²⁰¹Tl (4.0±2.3), and 4-h GIK-²⁰¹Tl (5.6±2.7) were lower than that of ^99mTc-PYP (7.5 ±4.1) (p<0.05), and the ES had significantly declined 6 months later on ²⁰¹Tl (3.5±2.8) (p<0.05). Although the RDS of R-²⁰¹Tl (11.3±7.9) and 4-h GIK-²⁰¹Tl (11.2±6.3) were greater than at the 6-month ²⁰¹Tl (7.1±6.5), immediate GIK-²⁰¹Tl (7.4±6.5) was equivalent to follow-up ²⁰¹Tl. The sensitivity of immediate GIK-²⁰¹Tl was highest among the imaging methods. Conclusion To detect myocardial viability after AMI, early imaging with GIK-²⁰¹Tl is more useful than resting ²⁰¹Tl imaging. (Circ J 2004; 68: 348 - 354)