Simultaneous lidar measurements of the mesospheric sodium layer were made at the Air Force Geophysics Laboratory (AFGL) and University of Illinois at Urbana-Champaign (UIUC) on the nights of August 28, October 9 and October 16, 1985. On these nights the layer centroid height was 1 to 3 km lower at AFGL. In October the altitude of the layer bottomside was near 80 km at AFGL compared to 85 km at UIUC. Gravity wave activity was present in the layer at both sites on all 3 nights. In October, the wave amplitudes were significantly larger at AFGL. The sodium profiles were compared with similar measurements obtained at the Goddard Space Flight Center in October 1981 and at the White Sands Missile Range in October 1984. The data from all four sites indicate that there is a significant longitudinal variation in the layer structure between 70¿ W and 90¿ W. It is suggested that these structural differences may be due to dynamics. The most promising mechanism capable of procuding the large 1 to 3 km differences in the layer centroid heights between UIUC and AFGL is a small difference in the mean vertical wind velocity. Because the typical residence time for a sodium atom in the layer is approximately 5.8 days, a 0.4 cm s-1 difference in the mean vertical wind velocities at the two sites is capable of inducing a 2-km difference in the centroid height of the sodium layer.