Objectives: Nuclear magnetic resonance (MR) phase velocity encoding techniques were developed for assessment of three-dimensional blood flow patterns and regional blood flows in infrarenal aortic aneurysms in vivo. Methods: Twenty patients with abdominal aortic aneurysms were investigated before elective surgery with a 1.5 Tesla MR-scanner. Standard multislice spin-echo sequences were used for aneurysm imaging. A flow-adjusted gradients sequence (FLAG) provided three-dimensional vector plots depicting local blood flow velocities as functions of time and anatomical position. Computer-generated animated presentations of the vectors were developed to ease data analysis and interpretation. Results: The blood flow patterns in infrarenal aortic aneurysms were much more complex than previously believed. Their main characteristics were simultaneous breakdown of the antegrade flow and creation of major retrograde flow components. Major pattern determinants included inlet geometry and lumen morphology, especially presence or absence of a thrombus. Conclusions: The frictional forces generated within the lumen as a result of the breakdown of laminar flows are probably translated to the aneurysm wall and contribute to thrombus formation, aneurysm growth and risk of rupture.