INTRODUCTION Evolution of aortic intramural hematoma (IMH) over time ranges from resolution to degeneration and is difficult to predict. We sought to measure differences in contrast attenuation between arterial and delayed phase computed tomographic angiography (CTA) METHODS IMH institutional data were gathered for 2005-2020. Hounsfield unit ratio (HUR) was measured as hematoma Hounsfield unit (HU) on delayed phase divided by arterial phase on CTA. Aortic growth and effect of HUR were determined using a linear mixed effects model. Freedom from adverse aortic event, defined as the composite of intervention, recurrence of symptoms, and rupture, was determined using Kaplan-Meier analysis. RESULTS IMH occurred in 73 patients; 27 met the inclusion criteria (Table). HUR ranged from 0.38-1.92 (mean:0.98). Baseline aortic diameter growth independent of HUR measurement was 0.49mm/yr (95%CI:-1.23-2.2), however, in patients with HUR > 1 (increased enhancement on delays) aortic diameter grew 5.05mm/yr per HUR unit (95%CI:0.56-9.56;p=0.028). Conversely, in patients with HUR < 1 (diminished enhancement on delays), the aorta shrank consistent with IMH resolution. Aortic adverse events occurred in 14(51.9%) patients at a median of 19 days (IQR:1-95 days); 7(25.9%) patients had recurrence of symptoms, 8(29.6%) required intervention, 5(18.5%) progressed to dissection, and 1(3.7%) had aortic rupture. There was a trend toward an association between higher HUR and composite adverse aortic events (HR 3.21 per 1-unit HUR;95%CI:0.60-17.3;p=0.18). CONCLUSION Increased HUR is associated with increased aortic growth and a trend toward adverse aortic events. Diminished delayed phase enhancement may predict partial or complete IMH resolution. HUR can be used to guide IMH surveillance and treatment. Evolution of aortic intramural hematoma (IMH) over time ranges from resolution to degeneration and is difficult to predict. We sought to measure differences in contrast attenuation between arterial and delayed phase computed tomographic angiography (CTA) IMH institutional data were gathered for 2005-2020. Hounsfield unit ratio (HUR) was measured as hematoma Hounsfield unit (HU) on delayed phase divided by arterial phase on CTA. Aortic growth and effect of HUR were determined using a linear mixed effects model. Freedom from adverse aortic event, defined as the composite of intervention, recurrence of symptoms, and rupture, was determined using Kaplan-Meier analysis. IMH occurred in 73 patients; 27 met the inclusion criteria (Table). HUR ranged from 0.38-1.92 (mean:0.98). Baseline aortic diameter growth independent of HUR measurement was 0.49mm/yr (95%CI:-1.23-2.2), however, in patients with HUR > 1 (increased enhancement on delays) aortic diameter grew 5.05mm/yr per HUR unit (95%CI:0.56-9.56;p=0.028). Conversely, in patients with HUR < 1 (diminished enhancement on delays), the aorta shrank consistent with IMH resolution. Aortic adverse events occurred in 14(51.9%) patients at a median of 19 days (IQR:1-95 days); 7(25.9%) patients had recurrence of symptoms, 8(29.6%) required intervention, 5(18.5%) progressed to dissection, and 1(3.7%) had aortic rupture. There was a trend toward an association between higher HUR and composite adverse aortic events (HR 3.21 per 1-unit HUR;95%CI:0.60-17.3;p=0.18). Increased HUR is associated with increased aortic growth and a trend toward adverse aortic events. Diminished delayed phase enhancement may predict partial or complete IMH resolution. HUR can be used to guide IMH surveillance and treatment.