Factors determining the onset and decay of the Polar Mesosphere Summer Echo (PMSE) season in 1997 are examined. PMSE from the ESRAD radar at 68 ø N, 20 o E are compared with simultaneous observations of stratospheric winds, upper-mesosphere temperature, upper mesosphere water vapour and noctilucent clouds. There was a 3-week delay from the start of summer, defined by temperature or stratospheric winds, to the start of PMSE. At the end of the summer, the return to winter conditions was simultaneous in all three parameters. Noctilucent clouds at 55-60 ø N were confined to the period of lowest temperatures, when PMSE were also present throughout the day. Water vapour observations show an increase during the summer, possibly taking place in late May, at about the time PMSE first appeared. It seems likely that the start of the PMSE season is determined by water vapour availability, while the end of the season is determined by temperature.