A validation study is carried out for the Geosat altimeter in the Gulf Stream meander and ring region. A synthetic geoid, containing the medium scales of interest for mesoscale research, is constructed by subtracting an independent estimate of the oceanographic mean dynamic topography from the altimetric sea surface. The oceanographic mean dynamic topography which is concurrent with the altimetric mean is derived from a year of real-time nowcasts and forecasts (Gulfcast) based on assimilating a composite data set into a dynamical model. Using the synthetic geoid, absolute dynamic topography from Geosat and Gulfcast for a year and 4 months in 1988 and 1989 compare very well in signal shape and feature locations. Sixteen cases of comparison of dynamic heights derived from AXBTs along Geosat tracks in 1987 are made with Geosat absolute topography. The rms difference is less than 10 cm. Thus the comparison of these two independent data sets validates the altimeter and the synthetic geoid within the noise and error levels of the system. ¿1991 American Geophysical Union