Two major outlet glaciers in East Greenland have suddenly begun to accelerate and retreat. The speeds of Kangerdlugssuaq and Helheim remained steady during the 1990s despite progressive and substantial thinning, but have abruptly increased within the last two years, more than doubling ice flux to the ocean. Had it been an isolated example, the comparable 1998 speed-up of Jakobshavn Isbræ in West Greenland might have been explained simply by its chance retreat past a pinning point. Now that two further Greenland outlets have exhibited similar behavior, a common process seems likely. A remarkable correspondence in the inter-annual patterns of speed and ice-front variation between Kangerdlugssuaq and Helheim implies a significant sensitivity to regional environmental factors. The period of continued warming and thinning appears to have primed these glaciers for a step-change in dynamics not included in current models. We should expect further Greenland outlet glaciers to follow suit.