© 2018 Society for Vascular Surgery Objective: The purpose of the Society for Vascular Surgery Vascular Quality Initiative thoracic endovascular aortic repair (TEVAR) for dissection project is to assess the effectiveness of TEVAR for type B dissection by evaluation in a prospective quality improvement registry. Here we describe the project cohort and 30-day outcomes of TEVAR for both acute dissection (AD) and chronic dissection (CD) patients and focus specifically on outcomes of uncomplicated AD patients based on timing of treatment. Methods: Summary statistics were performed comparing patients with AD (<30 days) and patients with CD. Both groups were further divided into those with complicated (ie, malperfusion or rupture) or uncomplicated presentation. Further subdivision of the uncomplicated AD patients into treatment at ≤48 hours, >48 hours to <7 days, ≥7 days to ≤14 days, and >14 days to <30 days was performed. Kaplan-Meier analysis was performed for 30-day survival and freedom from reintervention. Results: Data for 397 patients (204 AD patients and 193 CD patients) were collected from 40 institutions. Overall, AD patients were younger than CD patients (58.8 vs 62.2 years; P =.003). Technical success, including coverage of the primary entry tear, was 98.0% for AD patients and 99.0% for CD patients, with a trend toward a higher 30-day mortality in AD patients (AD, 9.3%; CD, 5.2%; P =.126). Any degree of procedure-related spinal cord ischemia occurred in 4.4% of AD patients vs 2.1% of CD patients (P =.261), with a deficit at discharge in 3.4% of AD patients vs 0.5% of CD patients (P =.068). Disabling stroke occurred in 2.5% of AD patients vs 1.6% of CD patients (P =.725); retrograde type A dissection occurred in 1.1% of AD patients vs 2.6% of CD patients (P =.412). There was a trend toward a lower freedom from reintervention in AD patients (90.7% vs 94.8%; P =.13). In uncomplicated AD patients, rapid aortic expansion was more common in the treatment groups of ≥7 days to ≤14 days and >14 days to <30 days compared with those treated within 7 days of dissection (P =.042). The uncomplicated AD cohorts based on timing of treatment were otherwise similar in demographics and presentation, with no significant differences in 30-day mortality or serious complications, such as spinal cord ischemia, stroke, or retrograde type A dissection. The 30-day reintervention rate for uncomplicated AD patients was 5.8%, with no apparent differences in reintervention rates according to timing of treatment of initial TEVAR. Conclusions: As expected, AD patients demonstrated a trend toward a higher 30-day mortality and lower freedom from reintervention compared with CD patients. Mortality at 30 days after TEVAR for uncomplicated AD was 5.8%, and there were no clear patterns in mortality or reintervention based on timing of treatment. Further study and evaluation at longer follow-up are needed to determine the impact of timing of intervention in uncomplicated AD patients.