We report on the first annual cycle of mesopause-region (82--105 km) temperature profiles at a tropical latitude site, the Arecibo Observatory (18.35¿N, 66.75¿W), measured by potassium resonance lidar. Based on 74 night-time temperature profiles of observing periods greater than 5 hours we observe a mean temperature profile with a mesopause altitude of 98 km and temperature of 193 K. The annual mean nighttime temperature profile is similar to that seen at mid-latitudes, but there are striking differences in seasonal structure. The mesopause region mean seasonal structure at Arecibo experiences a temperature minimum of 176 K at 100 km during mid-summer. The winter period minimum of about 190 K is found mostly in a broad flat distribution between 90 and 95 km, and less frequently near 100 km.