We report on observations of noctilucent clouds (NLCs) by a ground-based lidar located in northern Norway at 69¿N, 16¿E. The ALOMAR Rayleigh/Mie/Raman (RMR) lidar conducted measurements of the Arctic middle atmosphere from 1 June to 15 August during each year from 1997 to 2001. This data set contains 1122 hours of lidar observations whereof 408 hours include NLC signatures. The interannual variation of the NLC occurrence frequency shows a decrease of strong NLCs, while weak NLCs occur more frequent. The seasonal variation of the NLC occurrence shows a well pronounced core period where NLCs appeared during 43% of the time. The basic properties of NLCs are characterized by three parameters: maximum value of the volume backscatter coefficient ¿max (≡brightness), centroid altitude zc, and half width δz (≡thickness). A typical NLC above ALOMAR during the 5-year period reported here owns a brightness of ¿max = 9.6 ¿ 10-10 m-1 sr-1, an altitude of zc = 83.3 km, and a thickness of δz = 1.2 km. The interannual variation of the parameters shows a decrease of the brightness, an increase of the altitude, and a nearly constant thickness, while seasonal variability is higher than these interannual changes. During the core period, the NLCs are noticeably brighter than at the beginning as well as the end of the season. Altitude and thickness of NLCs decrease during the season.