Polar mesosphere summer echoes (PMSE) are strong enhancements of received signal power at very high radar frequencies occurring at altitudes between about 80 and 95km at polar latitudes during summer. PMSE are caused by inhomogeneities in the electron density of the radar Bragg scale within the plasma of the cold summer mesopause region in the presence of negatively charged ice particles. Thus the occurrence of PMSE contains information about mesospheric temperature and water vapour content but also depends on the ionisation due to solar wave radiation and precipitating high energetic particles. Continuous observations of PMSE have been carried out on the North-Norwegian island Andya (69.3°N, 16.0°E) since 1994. Seasonal mean values of PMSE occurrence for the time period June/July from 20 years of observations have been analysed for long term variations. The PMSE occurrence rate is positively correlated with the solar Lyman α radiation (however low significance level) and the geomagnetic Ap index. After elimination of the solar and geomagnetically induced parts the PMSE data show a significant positive trend during the time interval from 1994 until 2015. This positive PMSE trend could be caused by an increasing trend of the mesospheric water vapor and/or a decreasing temperature trend.