Previous satellite measurements have provided nearly complete seasonal and geographic coverage of polar mesospheric clouds (PMCs), but previous data sets have not been able to evaluate changes in PMC behavior on decadal timescales. The Solar Backscattered Ultraviolet (SBUV) series of ozone measuring instruments have been flying continuously since 1978. While the instrument design is not optimized for PMC detection, the radiance data can be analyzed to examine the occurrence frequency and intensity of relatively bright PMCs. In this paper, we present PMC results from five SBUV and SBUV/2 instruments covering more than 23 years (1978--2002), starting just before the maximum of solar cycle 21 and extending through the maximum of solar cycle 23. The overlapping data sets from nearly identical instruments give an accurate picture of long-term variations. Multiple linear regression fits are used to examine solar and secular correlations. PMC occurrence frequency is anticorrelated with solar Lyman alpha irradiance, with an approximate 0.5-year phase lag in the Northern Hemisphere (Rsolar = -0.87) and no phase lag in the Southern Hemisphere (Rsolar = -0.65). The distribution of cloud brightness by season appears to be changing over time. When the PMC brightness for each season is characterized using an exponential cumulative distribution function, the exponent decreases in magnitude by a factor of 2 from 1978 to 2002 in the Southern Hemisphere (Rtime = +0.85). This implies an increase in the relative proportion of the brightest PMCs. The secular brightness trend is less significant in the Northern Hemisphere (Rtime = +0.58). We discuss possible origins for these changes.