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We estimate the mass balance of eight drainage basins in West Antarctica from the Gravity Recovery and Climate Experiment (GRACE) data (GFZ RL04, GSM Level 2) using a constrained inverse-gravimetric approach. We consider InSAR observations of ice surface velocity as an indication of mass change, assuming that large mass loss occurs in areas of fast glacier flow. From these mass distribution functions we construct forward models of the geoid height change and their spatial correlations for each drainage basin. Then the difference between the GRACE data and the forward model is minimized by adjusting the total amount of mass change within each drainage basin. To overcome the ambiguity inherent in this inverse problem, we constrain its solution by including a priori estimates based on the InSAR mass-budget method. However, unconstrained (GRACE only) mass-change estimates can be recovered for three to four combined drainage basins. Differences between GRACE and InSAR values exist mainly for the Pine Island Glacier and Getz Ice Shelf region, resulting in a lower unconstrained GRACE total of −91.0 ± 3.5 Gt/yr (for the years 2002–2008) compared to the InSAR estimate of −116.6 ± 19.0 Gt/yr (outflow measurement for the years 1992, 1996, and 2006). There is evidence that this difference arises from anomalously large accumulation within the GRACE time interval (August 2002 to August 2008) in the Amundsen Sea sector and possibly from an overestimation of ice thickness for parts of the Bellinghausen Sea sector underlying the InSAR mass-budget values.