Radiocarbon-dated glacial-geologic evidence documents an abrupt advance of the northern margin of the Labrador sector of the Laurentide Ice Sheet during the last deglaciation. Ice-flow directional indicators, together with ice-marginal features found onshore and offshore, delimit an ice stream that advanced north-northeast >300 km, crossed the mouth of Hudson Strait and outer Frobisher Bay, and overran summits ∼400 m above sea level on outer Hall Peninsula, southeast Baffin Island. Fifty-eight 14C ages, including 27 accelerator mass spectrometry ages on single shells, indicate that the advance was extremely rapid. The entire advance-retreat cycle took place in an ∼300 yr (14C) interval, 9.9-9.6 ka. At its maximum extent, the ice stream supported a calving margin >200 km long terminating in open water ∼500 m deep, implying a massive iceberg release. Marine evidence for the outflow is preserved along the Labrador Sea shelf as thick carbonate-rich glacial-marine drift but has not been recognized farther east in the North Atlantic. Either the discharge of icebergs was insufficient to produce a trans-North Atlantic, carbonate-rich (Heinrich) layer, or the icebergs tracked southward where they encountered warming sea-surface temperatures. Although the advance seems to have been coincident with a brief interval of minor regional cooling, it failed to trigger a significant climate perturbation, suggesting that the ocean-atmosphere system had attained its stable interglacial condition by that time.