Purpose: To validate a noninvasive quantitative MRI technique, the K i perfusion method, for myocardial perfusion in humans using 13N-ammonia PEXT as a reference method. Materials and Methods: Ten healthy males (64 ± 8 years) were examined with combined PET and MRI perfusion imaging at rest and during stress induced by dipyridamole in order to determine the myocardial perfusion reserve. Myocardial and blood time concentration curves obtained by Gd-DTPA-enhaneed MRI and 13N-ammonia PET were fitted by a two-compartment perfusion model. Results: Mean perfusion values (±SD) derived from the MRI method at rest and at hyperemia were 80 ± 20 and 183 ± 56 mL/min/100 g, respectively. The same data for PET were 71 ± 16 and 203 ± 67 mL/min/100 g. A linear relationship was observed between MRI and PET-derived myocardial perfusion reserve for regional and global data. Linear regression for the global absolute perfusion reserve gave a correlation coefficient of 0.96 (P < 0.004, y=0.83x-6.9). A good agreement between the two methods to determine low or high perfusion reserves was found. Conclusion: Our data provide validation of the perfusion marker Ki derived by the MRI method as a quantitative marker for myocardial perfusion in healthy humans.