We have routinely observed mesospheric Na layers since November 1991 with a Na ground-based lidar above Tokyo Metropolitan University (TMU) at Hachioji, Tokyo (35.6 ¿N, 139.4 ¿E) in mid-latitude. In the past, the sporadic Na layers have been observed commonly at low- and high-latitude lidar sites [Kwon et al., 1988; Batista et al., 1989; von Zahn et al., 1987> but rarely observed at mid-latitude sites [Senft et al., 1989>. Contrary to expectations, we could observe more than 100 events of the sporadic Na layers for two years from November 1991 until October 1993 in this mid-latitude lidar site, and the large sporadic Na layers had been observed especially during spring and summer. Most of these events were accompanied by sporadic E layers and the most enhancement of the sporadic E layers preceded that of the sporadic Na layers by 15 to 30 minutes. ¿ American Geophysical Union 1995