Measurements of the lake level of Lake Langisj¿r at the SW edge of the Vatnaj¿kull ice cap indicate a tilt of 0.26¿0.06 &mgr;rad/year away from the ice cap in the years 1959--1991. The tilt is too large to be explained as an elastic Earth response to ice retreat this century, or to be caused by change in the gravitational pull of the ice cap, but it can be explained by sub-lithospheric viscous adjustment. Regional subsidence in historical times in SE Iceland can similarly be attributed to viscous adjustment resulting from the increased load of Vatnaj¿kull during the Little Ice Age. The inferred sub-lithospheric viscosity is 1¿1018--5¿1019 Pa s. ¿ American Geophysical Union 1992