Lidar measurements of potassium temperatures and number densities were performed in the upper mesosphere/lower thermosphere at Spitsbergen (78°N) in 2001–2003. The cumulative data set covers the season from March until September and altitudes from 80 to 110 km (less in summer). The seasonal variation of K densities shows a maximum from early April until late September, similar to other latitudes but opposite to other metals. Temperatures are derived from the Doppler broadened D1 line of potassium with a typical resolution of 1 km and 1 h, respectively. Systematic temperature uncertainties are approximately ±3 K. Statistical errors depend on integration times and are typically ±0.5–3 K for 1 d and 1 h, respectively. The K-lidar temperatures are generally consistent with simultaneous and colocated in situ measurements by falling spheres. Temperatures show variability on all detectable scales. The magnitude of the fluctuations varies little with season and is remarkably similar to analog observations at lower latitudes (54°N). The summer mesopause is located at ∼90 km and is as cold as ∼120 K. In winter the mesopause is significantly higher (∼100–105 km), warmer (∼180–190 K), and less pronounced. Lowest temperatures are detected at the mesopause around 4 July (day of year = 185) which is approximately 2 weeks after astronomical midsummer. Large differences of more than ±20 K occur between mean K temperatures and empirical climatologies. The lower thermosphere is significantly colder compared to general circulation models.