The parameters of a single-mode lognormal size distribution for stratospheric aerosol are obtained from two-wavelength lidar measurements. The method is based on the measurement of the aerosol optical thickness, tau(A)(lambda), and integrated backscattering coefficient, BI(lambda), at 351 and 580 nn. The aerosol particle median radius, r(m), and distribution width, sigma, are obtained by comparing experimental and computed values for tau(A)(351)/BI(351), tau(A)(580)/BI(580) and BI(580)/BI(351). The method has been applied to three nights of measurement (27 April 1992, 31 May 1993 and 7 March 1994) in the aftermath of the Mount Pinatubo eruption. The time lag between the eruption and the measurements suggests that the first measurement is representative of a fresh volcanic aerosol, while the second and third measurements are representative of an old and depleted aerosol. The measurements performed on 27 April 1992, 31 May 1993 and 7 March 1994 lead, respectively, to r(m) = 0.31 +/- 0.05 mu m and sigma = 1.3 +/= 0.1, r(m) = 0.16 +/- 0.04 mu m and sigma = 1.9 +/- 0.1, r(m) = 0.21 +/- 0.06 mu m and sigma = 1.8 +/- 0.1. The aerosol mass content and number density for the three nights of measurement have also been determined.