Rainfall estimates from the Tropical Rainfall Measuring Mission (TRMM) Precipitation Radar (PR) facility algorithm over the eastern Pacific are lower than those from the TRMM Microwave Imager (TMI) facility algorithm and a PR-consistent advanced microwave radiometer algorithm, while they are in good agreement over the western Pacific. We investigate the consistency between TMI-observed brightness temperatures (TBs) for the 19 GHz channel and those simulated from the PR rainfall estimates using a radiative transfer model. Discrepancies between observed TBs and simulated TBs from the PR are larger for the eastern Pacific than for the western Pacific, indicating that PR underestimates rainfall over the eastern Pacific. It is hypothesized that drop size distributions (DSDs) in the eastern Pacific have stronger maritimity (i.e., more small to medium sized raindrops) than the initial DSD model of the PR algorithm, representative of the "data-rich" western Pacific, leading to underestimation of rain by the PR. Differences in the adjustable parameter of the PR algorithm, implying changes in the DSD, and the vertical structure of PR-observed reflectivity over the western and eastern Pacific support the hypothesis. Copyright 2008 by the American Geophysical Union.