The eruption of Mt. Pinatubo (15¿N, 122¿E) on June 15 and 16, 1991, placed a large amount of SO2 and crustal material in the stratosphere. Based on measurements of decreases of stratospheric ozone after previous volcanic eruptions, it was expected that the aerosols deposited into the stratosphere (both directly and as a result of SO2 conversion into particulate sulfate) by this eruption would give rise to significant ozone depletions. To check for such an effect, ozone profiles obtained from ECC sondes before and after the eruption at Brazzaville, Congo (4¿S, 15¿E), and Ascension Island (8¿S, 14¿W), are examined. Aerosol profiles determined from a lidar system in the western Pacific (4¿--6¿N, 125dE) show that most of the material injected into the stratosphere is located between 18 and 28 km with highest amounts at 24--25 km. For the period 3--6 months after the eruption, decreases in ozone are found at 16 to 29 km, with peak decreases as large as 20% found at 24 km. Integrated between 16 and 28 km, a decrease of 13--20 Dobson units is observed when the ozonesonde data after the Pinatubo eruption are compared with those prior to the eruption. The altitude at which the most pronounced ozone decrease is found strongly correlates with peak aerosol loading determined by the lidar. In addition, a small increase in ozone density is found above about 28 km. Mechanisms that might explain the results such as heterogeneous chemistry, radiative effects, and dynamics are discussed. ¿ American Geophysical Union 1992