Lithospheric flexure seaward of deep ocean trenches is evident in Seasat altimeter observations of the marine geoid. In fact, mechanical models of lithospheric flexure can be tested directly on the Seasat altimeter data. We have used a simple elastic model for the oceanic lithosphere and, after least squares adjustments, have recovered estimates of model parameters including outer rise (OR) amplitude, OR wavelength, and effective lithospheric thickness. Effective lithospheric thicknesses have been recovered for six regions: the Mariana, the Kuril, the Philippine, the Aleutian, the Izu-Bonin, and the Middle America OR's. These results support the proposition that effective thickness Te increases with age of lithosphere in approximate accord with the relation Te≂C⋅age1/2 where C≂4 km m.y.-12. In fact, our altimetric results agree more closely with this relation than do published results based on bathymetric data. The close agreement with the agreement with the thickness-age relation suggests that there is no longer any need to assume that significant horizontal compression acts across the Kuril, Marianas, and Izu-Bonin trenches. This thickness-age relation implies that flexural strength of the oceanic lithosphere is temperature controlled.