The 5 km debris-covered tongue of Miage Glacier has been studied to explore the effect of the debris cover on patterns of thickness change over an 86-year period (1913-1999). Changes in surface elevation and volume for four intervals of eight to 44 years' duration have been calculated from comparisons of digital terrain models (DTMs) derived from cartographic and topographic surveys. Thickness changes over successive periods show that parts of the ablation zone have thickened while other parts have thinned or maintained stable elevations. Zones of thickening migrated downstream to cause small advances of the terminus on two occasions. In contrast to nearby uncovered glaciers, Miage Glacier increased in volume over the entire period. Our results indicate that decadal-scale thickness changes are forced primarily by mass flux perturbations in synchrony with uncovered glaciers, and not by debris-mantle insulation. Century-scale volume and thickness changes differ from uncovered glaciers, due to the conservation of ice beneath the debris cover during prolonged periods of thinning.