The Medium Earth Orbit (MEO) region is becoming increasingly exploited as the number of navigation constellations grows with the advent of the European GALILEO and the Chinese COMPASS systems. There is the need for an effective disposal strategy of satellites at end-of-life able to prevent any possible damage of operational satellites. This strategy has to take into account the known instability of nearly circular disposal orbits in MEO. These orbits show an increase of the eccentricity that could lead to dangerous crossings of the operational orbits.The nature of this instability, linked to luni-solar resonances, is briefly recalled. Then the effect of different disposal strategies on the long term evolution of theMEO environment is analyzed. It is shown how the disposal of satellites at end-of-life into well separated storage zones, above the constellations operational orbits, is capable of limiting the collision risk for operational satellites to very low values for long time spans (200 years). Moreover, a disposal into eccentric orbits not only keeps the collision risk to very low values, but also has the advantage of reducing the lifetime of the disposed satellites, thus freeing the orbital environment from the uncontrolled spacecraft.