The source of the buoyancy that supports the Hawaiian Swell is not well understood. The swell may be supported by replacement of negatively buoyant lithosphere with buoyant mantle or by emplacement of buoyant material beneath lithosphere of normal thickness. These mechanisms can be distinguished by examining the quadratic relationship between geoid height and bathymetry. At the Hawaiian Swell, the curvature of the geoid height vs. swell topography relationship is negative, indicating that the swell is supported by thinned lithosphere. The magnitude of the curvature suggests that the mantle filling in the region of thinned lithosphere is both thermally and chemically buoyant.¿ 1997 American Geophysical Union