It is shown that the equivalent blowing velocity of a three‐dimensional boundary layer can be either positive or negative near separation depending on the geometry of the flow. The situation is illustrated by a computation of the boundary layer in the planes of symmetry of a thin ellipsoid moving in the direction of its major axis. This result is of importance in estimating the effect of the boundary layer on the external flow and is distinct from the two‐dimensional problem where the blowing velocity always appears to be positive near separation.