Hyperbaric oxygen treatment (HBO) could transiently reverse hypoxia during acute myocardial infarction (AMI). In order to evaluate whether early HBO can identify viable segments after AMI, improvement of wall motion score index (WMSI) after HBO was compared to dobutamine stress echocardiography (DSE). Thirty-one patients with first AMI treated with thrombolysis received 100% oxygen at 2 technical atmospheres for 1 h within 24 h of the onset of chest pain. All patients underwent echocardiography before and after HBO and during DSE. Improvements in WMSI after HBO, as well as during DSE were considered as proof of viability. Total of 186 akinetic segments were detected before HBO. Functional recovery was defined at 73 after HBO and 113 segments were fixed. Eighty-one segments improved contraction with DSE. WMSI improved before HBO compared to the one after HBO (1.79 vs. 1.65, p = 0.024) and DSE (1.79 vs. 1.60, p < 0.001). Close relationship between WMSI after HBO and DSE was found (r = 0.417, p = 0.022). Sensitivity and specificity of HBO for viability were 73% and 85%, respectively. HBO may identify viable myocardium as early as day one after AMI. The highest number of responding segments was detected in patients who received HBO within shortest intervals following the onset of chest pain.