Dense stratospheric aerosol layers originating from the volcanic eruptions of Mt. Pinatubo in the Philippines on June 15, 1991 were observed at the Wakkanai radio observatory (45.4¿N, 141.7¿E) of the Communications Research Laboratory (CRL) using a Nd:YAG lidar system. These observations started in late August 1991, and the dense layer was first detected in early October 1991. The backscattering of the aerosol layers varied greatly in October and November 1991 then stabilized in December 1991. The integrated back-scattering coefficient (IBC) at the beginning of 1992 was smaller than the values observed in October and November 1991. The IBC increased slightly during 1992, but began to decrease in 1993. The estimated surface area density of aerosol particles increased one to two orders of magnitude in late 1991 and early 1992 to about 10-7 cm2/cm3. The IBC and surface area started to decrease sooner at higher altitudes: in May 1992 from 25 to 30 km, in November 1992 from 20 to 25 km, and in February 1993 from 13 to 20 km in altitude. Calculation by Prather [1992> indicates that our estimated surface area correspond to about 30 to 50% less NOx. Almost same degree of decrease in NO2 was observed by Koike et al. [1993>.