Gr¿msv¿tn is a subglacial volcano, under the Vatnaj¿kull ice cap at the center of the Icelandic hotspot. This highly active volcano erupted in December 1998. GPS measurements at a single station on a nunatak at the caldera rim were made 7 times during 1992--2001. The measurements prior to the 1998 eruption reveal pre-eruption inflation, but subsidence of more than 15 cm was measured during the eruption. Following the eruption, re-inflation occurred initially, at a rate of 20 mm/month, then declined to 5 mm/month. Measurements were fitted to a Mogi model, assuming that the source was located under the center of the Gr¿msv¿tn caldera complex. Results indicate a source depth of at least 1.6-km. The calculated amount of magma outflow during the eruption is comparable to field estimates of the erupted volume. Gr¿msv¿tn continues to inflate, but has not reached its 1998 pre-eruption level.