We use lidar measurements at wavelengths of 10.59 and 0.694 μm, together with contemporaneous in situ measurements of aerosol size distribution width, to retrieve the time-height evolution of lognormal median radius and number density for stratospheric aerosols over the central United States during the first two years after the eruption of Mount Pinatubo. Comparisons with the in situ measurements of median radius and number density are favorable only if the 10.59-μm backscatter is doubled, indicating a calibration bias in NOAA's climatological data set. The time histories of lidar-retrieved parameters at lower altitudes indicate that material from a new source appeared in the stratospheric aerosol cloud in the fall of 1992, about 500 days after the eruption of Mount Pinatubo. ¿ 1997 American Geophysical Union