Abdominal aortic aneurysms occur in as much as 2-3% of the population, and their rupture produces a mortality rate of 78-94% (1), causing 15,000 deaths per year in the U.S. alone. As an investigation into the mechanical factors that lead to aneurysm rupture, flow field measurements are presented for steady flow through a range of aneurysm sizes and Reynolds numbers. Seven rigid symmetric models of aneurysms were constructed with uniform lengths of 4d and diameters that ranged from 1.4 to 3.3d, where d is the inner diameter of the undilated entrance tube. Color Doppler flow imaging was used to visualize the flow fields, while laser Doppler velocimetry was used to quantify the flow field velocities and to determine critical Reynolds numbers for the onset of, and complete transition to, turbulent flow. Estimates of mean and peak wall shear stresses were derived from velocity measurements. Flow in these models varied from fully laminar to fully turbulent over the range of Reynolds numbers corresponding to in vivo flows. There was a large range over which the flow was intermittently turbulent. High wall shear occurred in the models when the flow was turbulent, suggesting that turbulence in in vivo aneurysms may contribute significantly to their risk of rupture.