Our interpretation of pollen and ostracode successions from four basins in south-central Illinois provides a new synthesis of palaeovegetation, palaeohydrology, and palaeoclimate for the period from the late Illinois Episode (about 130,000 years ago) to near the end of the Wisconsin Episode (about 25,000 years ago). Correlations of pollen biozones between Raymond, Pittsburg, and Bald Knob basins are the basis for identifying the late Illinois glacial, Sangamon interglacial, Wisconsin glacial, and Hudson interglacial episodes. Glacial episodes were identified primarily by the presence of Picea pollen and the ostracode Limnocythere friabilis, whereas interglacial episodes were identified by Quercus pollen and by the ostracodes Candona caudata and Heterocypris punctata. Within interglacial and glacial episodes, pollen and ostracode assemblages varied with changes in moisture balance. Local palaeohydrology was assessed primarily on the basis of environmental tolerance indices of ostracodes and the stable isotope (C,O) stratigraphy of ostracodal calcite. Regional moisture balance was assessed from multivariate analyses of the pollen successions. Three climatic regimes occurred during the Sangamon Episode. (1) One regime was characterised by precipitation exceeding evaporation that promoted basin overflow. This climate was inferred from the high percentages (generally >80%) of deciduous-forest pollen. Peaks in the abundance of Liquidambar and Fagus pollen indicate that winters may have been slightly warmer, and effective moisture slightly greater, than at present. (2) The second climatic regime was continental, similar to the present climate of Illinois in which precipitation is equal to or just less than evaporation. This climate is inferred from abundant Ambrosia pollen (40 to 60%) and abundant nektic (swimming) ostracode valves which suggest a shallow lake. These conditions probably developed in association with a 'heat-low' over the interior of North America during marine oxygen isotope stages 5e and 5c. Associated with the transition between the first two climates are fossils of the subtropical ostracode Heterocypris punctata and the giant tortoise Geochelone crassiscutata that suggest short periods in winter when polar low-pressure systems did not extend into Illinois as they do today. (3) The third climatic regime occurred during the transition from the Sangamon interglacial episode to the Wisconsin glacial episode. A severely continental climate is indicated by the heat-tolerant ostracode Pelocypris tuberculatum, variable ??18O values of ostracode valves, and high environmental tolerance index values for the ostracode assemblages. The weedy Chenopodiaceae and Amaranthaceae families grew on exposed mudflats. The tree pollen associated with this type of climate included low percentages of Picea and Liquidambar, an assemblage that has no modem analogue. We suggest that this transitional climatic regime was associated with the large-scale changes in the climate system during marine oxygen isotope stage 4.