We use ERS-2 and ENVISAT satellite radar altimetry to examine spatial and temporal changes in the rate of thinning of the Pine Island Glacier, West Antarctica, during the period 1995 to 2008. We show that the pattern of thinning has both accelerated and spread inland to encompass tributaries flowing into the central trunk of the glacier. Within the 5,400 km2 central trunk, the average rate of volume loss quadrupled from 2.6 ± 0.3 km3 yr-1 in 1995 to 10.1 ± 0.3 km3 yr-1 in 2006. The region of lightly grounded ice at the glacier terminus is extending upstream, and the changes inland are consistent with the effects of a prolonged disturbance to the ice flow, such as the effects of ocean-driven melting. If the acceleration continues at its present rate, the main trunk of PIG will be afloat within some 100 years, six times sooner than anticipated.