Ground deformation in volcanic regions can be a precursor to resumption of activity. Volcanic eruptions are typically brief periods of activity punctuating very long inter-eruptive periods. This makes hazard evaluation a dif?cult task for volcanoes with low-recurrence eruptive activity, which often are poorly monitored. As a result, analysis of inter-eruptive periods by use of remote sensing techniques can provide important information on precursory activity and improve volcano hazard assessment. In August–September 2010 Mt Sinabung, Indonesia, reawakened after at least 400 years of dormancy. The ground deformation before this eruption was investigated by use of differential interferometric synthetic aperture radar data obtained from Japanese ALOSPALSAR radar imagery between 05 January 2007 and 31 August 2010. Results from InSAR time series processing detected significant ground deformation (subsidence) at several locations on the Karo plateau, and uplift in the summit area of Mt Sinabung. The persistent scatterers density obtained by use of ALOS data is suf?cient to enable extraction of temporal and spatial patterns of the deformation. The surface deformation at the summit can be modeled by using a spherical point-source model. Source data are consistent with a very shallow (hydrothermal) reservoir, with a linear increase in overpressure before the 2010 Mt Sinabung eruption. Hydrothermal origin is consistent with seismicity, tiltmeters, and analysis of ash products collected during and after the 2010 eruption. These results support the potential of L-band interferometry for hazard assessment in poorly monitored and highly vegetated volcanic areas and also indicate that hazard assessment for Indonesian volcanoes could potentially be improved by identi?cation of precursory (inter-eruptive) uplift periods.