Background - A novel 3-dimensional left ventricular (LV) mapping system uses low-intensity magnetic field energy to determine the location of sensor- tipped electrode catheters within the LV. Using this system, we sought to distinguish between infarcted, ischemic, and normal myocardium by comparing LV electromechanical mapping data with myocardial perfusion imaging studies. Methods and Results - Unipolar voltage potentials and local endocardial shortening (LS) were measured in 18 patients (mean age, 58 ± 12 years) with symptomatic chronic angina having reversible and/or fixed myocardial perfusion defects on single photon emission computed tomography imaging studies using 201Tl at rest and 99mTc-sestamibi after adenosine stress. Overall, a significant difference in voltage potentials and LS values was found between groups (P < 0.001 for each comparison by ANOVA). The average voltage potentials (14.0 ± 2.0 mV) and LS values (12.5 ± 2.8%) were highest when measured in myocardial segments (n = 56) with normal perfusion and lowest (7.5 ± 3.4 mV and 3.4 ± 3.4%) when measured in myocardial segments with fixed perfusion defects (n = 20) (P < 0.0001). Myocardial segments with reversible perfusion defects (n = 66) had intermediate voltage amplitudes (12.0 ± 2.8 mV, P = 0.048 versus normal and P = 0.005 versus fixed segments) and LS values (10.3 ± 3.7%, P = 0.067 versus normal and P = 0.001 versus fixed segments). Conclusions - In patients with myocardial ischemia, LV mapping, compared with myocardial perfusion imaging, shows (1) mild reduction of endocardial voltage potentials and LS in segments with reversible perfusion defects and (2) profound electromechanical impairment in segments with fixed perfusion defects. Thus, such an LV mapping procedure may allow the detection on-line of myocardial viability in the catheterization laboratory.