Many current passive-microwave rain-retrieval methods are based on databases built off-line using cloud models: the models are used to simulate rain events, and radiative transfer calculations produce the associated brightness temperatures. The radiative transfer models depend on, among others, the distribution of hydrometeor sizes. The most popular raindrop size distribution (DSD) used in most models is Marshall and Palmer's <1948>. Improvements were proposed later by Sekhon and Srivastava <1971>, Willis and Tattelman <1989>, and Feingold and Levin <1986>. Using these DSD models, we study the dependence of forward radiative transfer calculations on the hydrometeor size distribution and quantify the uncertainty due to DSD variability. ¿ 2000 American Geophysical Union