LARGE uncertainties exist in the present knowledge of the mass budget of the Antarctic ice sheet, because of a lack of data on the rates of both ice outflow and snow accumulation1. Present estimates indicate that both the outflow and the net accumulation are approximately equal to 2,000 km3 of ice per year (equivalent to about 6 mm of sea level)2. The temporal variation of accumulation rate is central to determinations of the mass budget, because accumulation can change rapidly in response to short-term climate variations, whereas ice flow varies only on longer timescales. Here we present time series showing changes in the net rate of snow accumulation since 1806 along a 700-km segment of East Antarctica. The accumulation record was derived from the thicknesses of annual layers in ice cores, deduced from seasonal variations in oxygen isotope ratio and in ice-crust stratigraphy. We find a significant increase in the accumulation rate following a minimum around 1960, leading to recent rates that are about 20% above the long-term mean. If this recent increase is widespread, as suggested by shorter-term accumulation data from across a large part of Antarctica, the positive imbalance (5–25% of the mass input) shown in recent studies of the ice sheet's mass budget1 may have existed only since the late 1960s. We estimate that this increase in accumulation rate should contribute to a lowering of sea level of 1.0–1.2 mm per year.