Radar meteorologists have estimated, over a number of years, rainfall rate with ground-based radars. Since the 1970s, use of weather radar as an active technique to measure rainfall from space has been proposed, and a number of radar concepts, including modified radar altimeter design, were proposed. However, no attempt has yet been made to determine rain cell characteristics from available altimeter data. This paper presents a method to estimate rainfall rate and rain cell diameter from TOPEX/Poseidon dual frequency radar altimeter waveforms. The rain-affected waveforms are first selected using a criterion based on a departure from a normal Ku-C-band backscatter coefficient relationship and on a threshold of liquid water content estimated by the TOPEX microwave radiometer. The analytical representation of echo waveforms under rain conditions is used to model the effect of rain. A minimization procedure is then used to determine the model's best fit to the observed waveforms. Several case studies are presented to illustrate the method. These cases include squall lines, isolated light rain cells, and a tropical cyclone. The method, shown to give a high-resolution description of the rain distribution under various weather conditions, could be used to obtain global rain cell characteristics over the ocean.