Velocity measurements are reported for three-dimensional laminar separated airflow adjacent to a backward-facing step using two-component laser Doppler velocimeter. The backward-facing step, with a height of S ΒΌ 1:0 cm, is mounted in a rectangular duct that has an upstream height of h ΒΌ 0:98 cm, downstream height of H ΒΌ 2 cm, and a width of W ΒΌ 8 cm. This geometry provides an aspect ratio of AR ΒΌ 8 and an expansion ratio of ER ΒΌ 2.02. The flow measurements covered a Reynolds number range between 98:5 6 Re 6 525. Measurements of velocity distributions reveal that a swirling ''jet-like'' flow develops near the sidewall in the separating shear layer, and the impingement of that flow on the stepped wall causes a minimum to develop in the spanwise distribution of the reattachment region. Reverse and recirculation flow regions develop adjacent to both the sidewall and the step, and these regions increase in size as the Reynolds number increases. Velocity distributions that were measured at various planes downstream from the step are presented, and predictions compare favorably with these measurements. The results show some interesting flow behaviors that could not be deduced from two-dimensional studies.