Ozone profiles measured with a ground-based microwave instrument in years from 1989 through 1992 at latitude 34 ¿N, were searched for effects arising from the eruption of Mount Pinatubo in 1991. Between 20 and 24 km, ozone values after November 1991 were systematically less than in earlier years, and the deviation generally grew with time through May 1992. The minimum was 11 to 15% below values in earlier years, and occurred later than minima observed with ozonesondes at higher latitudes or predicted by some models. Although there was a steady increase in ozone levels in the 18--10 hPa range between February and May 1992, the highest values were not significantly above normal. Effects of the quasi-biennial oscillation on ozone values are much smaller than the observed changes. Colocated lidar observations show that stratospheric aerosol levels were steadily elevated from November 1991 through at least March 1992, with some intermittent activity in late August through October 1991. The main aerosol cloud thus arrived at the same time as depletion first appeared. ¿ 1998 American Geophysical Union