Injection of ethanol into the Vein of Marshall (VOM) has been employed to facilitate placement of a lateral mitral isthmus ablation line. The optimal dosage and delivery technique into the VOM remains undetermined. To characterize the relationship between the volume of ethanol injected into the VOM on left atrial infarct size. A retrospective review was performed of chart data, angiography, and electroanatomical mapping (EAM) of all patients who underwent ethanol injection into the VOM between 10/2016-9/2020. After PVI, patients underwent VOM cannulation (typically with a 1.5-2.5 mm over-the-wire balloon placed proximally in the vein). High density EAM (fill threshold ≤ 5 mm) was performed with a multielectrode array catheter pre- and post- each dose of ethanol. Pre-and post-injection voltage maps were available for review in 70 patients. A mean of 2.8 ± 1.2 cc of ethanol were injected, forming infarcts 5.9 ± 3.6 cm2 in size. Overall, amount of ethanol infused did not correlate with scar size (R2=0.0036). In 10 patients, a second dose of ethanol was given after EAM demonstrated insufficient infarct. In this group, the initial volume of ethanol injected was 1.9 ± 0.9 cc and total volume injected reached 4.1 ± 1.7 cc. Mean infarct size after first injection was 3.8 ± 2.7 cm2, vs 7.1 ± 4.1 cm2 after second injection (p<0.01). Improved extension of the scar area into the posterior wall was also seen. In this retrospective analysis, there did not appear to be a dose-dependent relationship between ethanol volume and infarct size. However, a second injection did lead to greater infarct area of the left atrium.