The 230Th ages and 234U/238U ratios were determined for Barbados corals that grew during periods of high sea level within the last 200,000 years. The similarity of the initial 234U/ 238U ratios of some of the corals to the modern marine value suggests that these samples are pristine and that the marine 234U/ 238U ratio 83,000 and 200,000 years ago was within 2 per mil of the modern value. The accuracies of the 230Th ages are evaluated on the basis of the 234U/238U values and a model of the behavior of uranium and thorium isotopes during diagenesis. For the last three interglacial and two intervening interstadial periods, sea level peaked at or after peaks in summer insolation in the Northern Hemisphere. This overall pattern supports the idea that glacial-interglacial cycles are caused by changes in Earth's orbital geometry. The sea-level drop at the end of the penultimate interglacial, the last interglacial, and a subsequent interstadial period lagged behind the decrease in insolation by 5,000 to 10,000 years.