The University of Hawaii's continuously cored borehole through the Midway Atoll coral cap provides a record of post–early Miocene eustasy. Subaerial exposure horizons are identified by light carbon isotope signatures and, in some instances, by lithologic criteria. Sea-level highstands are recorded by aggradational units of reef sediment that are bounded by these subaerial exposure horizons. Sea-level lowstands are constrained to fall no farther than a meteoric-marine diagenetic boundary observed in the core. This diagenetic boundary is defined by an offset in both oxygen and carbon isotope data, light isotopic composition being indicative of intermittent meteoric diagenesis. These data constrain the amplitudes of three post–early Miocene glacio-eustatic sea-level falls to be no more than 32, 43, and 52 m. Absolute sea-level elevations are estimated by constructing a subsidence model for Midway Atoll. Application of this analytic approach may provide amplitude constraints on eustasy elsewhere in the geologic record.