Joint operations of the ALOMAR Rayleigh/Mie/Raman (RMR) lidar and ALOMAR SOUSY radar (located at 69 ¿N latitude) let us obtain a first series of simultaneous and common-volume observations of noctilucent clouds (NLC) and polar mesosphere summer echoes (PMSE). We present the results of 22 examples of simultaneous NLC and PMSE observations made during the summers 1994 until 1997. These observations show a strong correlation of NLC with PMSE and they lead us to a classification (3 types) of the observed events. Furthermore, we conclude that, in all probability, there are common causes for the two types of layers. ¿ 1999 American Geophysical Union