The issue of the variability of coronal emission is a current problem of stellar physics. Properties of X-ray variability, amplitude and time scales give us information on the physics underlying the coronal emission. In this work we present results from a systematic analysis of X-ray variability of nearby M stars, mainly focused on medium and long term time scales. Taking advantage of archival data of ROSAT-PSPC observations, we explore time scales from days to months. Variability is present at all explored time scales and its amplitude is independent of stellar X-ray and visual luminosity. Results are compared with solar X-ray variability properties suggesting that in dM stars emitting Structures have a spatial distribution more homogenous that in the Sun. Analogous studies on dM stars of the Pleiades indicate that in this cluster the spread observed in the X-ray luminosity function can be explained in terms of variability.