The effect of shear flow on a concentrated block copolymer solution with lamellar microstructure was studied by in-situ small angle neutron scattering. Microstructural changes were determined from scattering measurements in all three principal directions (flow, velocity gradient, and neutral directions). The lamellae were found to align along the velocity direction but exhibited two orientations:  one parallel to the shearing surfaces and the other perpendicular to the shearing surfaces. The perfection of the perpendicular alignment was always significantly greater than that of the parallel alignment. In fact, our sample in the parallel alignment was, to a large extent, disordered (i.e., liquid-like). In some cases, we found transient, well-aligned, perpendicular lamellae that eventually degenerated to poorly ordered, parallel lamellae. In one case, a systematic increase in lamellar undulations was evident prior to disordering. These results are not in agreement with current theories, which only predict enhanced order in block copolymers under shear flow.