Near-infrared spectroscopy (NIRS) is widely used to record activation-related blood oxygenation changes in human brain tissue. However, the changes in the NIRS signal upon increased flow are influenced not only by the hemoglobin and oxyhemoglobin concentrations but also by changes in light scattering by various brain constituents. This paper points out the large contribution of flow-dependent red blood cell (RBC) aggregation as a cause of this altered light scattering, a phenomenon which has not previously been considered in the theoretical analysis of NIRS signals. Here, we show that RBCs, which constitute a major chromophore in the tissue, not only absorb light at hemoglobin molecules but also scatter it strongly at the cell membranes of aggregated RBCs, and that the blood optical density per se changes greatly with the size of the plasma gap, which varies according to flow. When local blood flow increases by 50%, the amount of the optical attenuation due to RBC dispersion/disaggregation (the flow effect) can reach 90% of the NIRS signal change for venous blood. The reasons why the optical signal due to blood oxygenation alone can be amount to less than 10% of the total are because the near-infrared lies in the most unfavorable range in the hemoglobin absorption spectrum for determining blood oxygenation, while the flow effect in the NIR range is large. We conclude that reported activation-related changes in brain blood oxygenation, at least in the peripheral region around the activation focus, based on NIRS can be mainly ascribed to the flow effect arising from RBC dispersion/disaggregation with increased flow in the venous system.