We used a sudden-expansion flow chamber to examine U-937 cell adhesion to unactivated and tumor necrosis factor (TNF)--activated human umbilical vein endothelial cells (HUVEC) in recirculating flow. For both unactivated and TNF--activated HUVEC, U-937 cells exhibited transient arrests within ~150 µm of flow reattachment. Few arrests occurred directly at the reattachment site. U-937 cell rolling was not observed. At all other locations within the recirculation zone, U-937 cells did not exhibit transient arrests or rolling. TNF- activation increased the frequency of U-937 cell arrests near reattachment but did not change the median arrest duration. Numerically simulated cell trajectories failed to predict attachment near the reattachment point. Deviations between experiment and theory may result from the nonspherical shape and deformability of U-937 cells. These results demonstrate that U-937 cell transient arrests occur preferentially in the vicinity of the reattachment point in recirculating flow. Possible mechanisms for adhesion include low shear stress, curved streamlines, fluid velocity components normal to the endothelium, and formation of larger contact areas.