Multitemporal glacier area mapping is a key element in accurately determining fresh water reserves, as well as providing an indicator of climate change. In Peru, the first glacier inventory was based on visual interpretation of aerial photos, requiring several years of effort. Landsat Thematic Mapper satellite imagery, on the other hand, provides an increasingly employed alternative for the monitoring of changes in glacier area and in other glaciological parameters. By means of Normalized Difference Snow Index (NDSI) computations on TM images, an estimate of the glacierized area in Cordillera Blanca (Peru) was carried out for 1987 (643±63 km²) and 1996 (600±61 km²). Compared to an estimate of 721 km² in 1970, it can be concluded that the glacier area has retreated in this massif by more than 15% in 25 years.