Based on 417 nights of lidar observation between 1990 and 1999 at Fort Collins, CO (41 ¿N, 105 ¿W), we tabulate an eight-year composite of monthly mean temperatures and Na densities in the mesopause region, and plot the associated altitude/month contours. The maximum and minimum mean mesopause temperatures were 192 K at 101 km and 178 K at 85.5 km on the 294th and 164th days of the year, respectively. The maximum and minimum monthly mean layer temperatures between 83 and 105 km were 205.8 K and 188.8 K, respectively, in November and July. When the episodic warming that peaked in 1993 is removed, the mean temperatures are typically lowered by ~4 K. Climatology supports a two-level thermal structure with mesopause occurring at higher altitudes in winter and lower altitudes in summer, and sharp transitions between them taking place in May and August. The maximum and minimum mean peak sodium densities were 4.88¿109 m-3 at 91 km and 1.49¿109 m-3 at 90 km on the 310th and 163th days of the year, respectively. ¿ 2000 American Geophysical Union