Measurements of significant wave height made by a pitch-roll buoy during the Joint Air-Sea Interaction Experiment (JASIN) are compared with measurements made at the same time by the SEASAT 1 radar altimeter as processed by the Jet Propulsion Laboratory SEASAT algorithms. Comparisons are made for eight occasions during which the significant wave height lay between 0.7 and 2.3 m. The radar altimeter is found to underestimate slightly the wave heights when compared with the pith-roll buoy. The mean ratio of the measurements is 0.96¿0.04, and the standard deviation of individual comparisons about this mean is 0.10.