Objective: The purpose of this study was to evaluate alterations in flow patterns in thoracic aortic dissections using 4-dimensional, flow-sensitive magnetic resonance imaging. Methods: This prospective study was conducted at 2 academic tertiary referral medical centers. Thirteen 4-dimensional flow magnetic resonance imaging studies were performed in 12 subjects (4 female, aged 25-71 years) with thoracic aortic dissection using 3.0T clinical scanners. Qualitative assessment of flow patterns in the true and false lumina was performed in consensus by 3 cardiovascular radiologists. Quantitative analysis included measurement of net flow, retrograde flow, peak flow, and time-to-peak flow in the true and false lumina in the ascending aorta, aortic arch, and descending aorta. Differences in flow through the true and false lumina at each analysis plane were compared with the 2-tailed, paired Student t test. Results: Flow patterns were significantly altered in association with different extents of disease, vessel dilatation, and post-therapeutic anatomy. Total flow per cardiac cycle and peak flow were higher in the true lumen than in the false lumen (P < .01). Retrograde flow was less in the true lumen than in the false lumen (P ≤ .01). Time-to-peak flow in the true lumen occurred later than in the false lumen (P = .05-.08). Conclusions: Four-dimensional, flow-sensitive magnetic resonance imaging at 3.0T provided qualitative and quantitative information on alterations of aortic flow in patients with thoracic aortic dissection. Future application of this magnetic resonance flow methodology may help provide insights into the pathophysiology and effects of flow alterations and establish prognostic indicators for the development of complications or aneurysm growth in patients with aortic dissection.