Sajama volcano, located in the Bolivian Altiplano, is the southernmost tropical glacier and, owing to its situation, approximately 100 km east of the Pacific coast, is well suited to study the El Ni¿o-Southern Oscillation phenomenon. Landsat data from 1972 to 1998 and a 1963 aerial photograph are used to monitor the snow line fluctuations on a selected part of Sajama volcano. We assume that a few months after the rainy season, the snow line is representative of the previous rainy season, if no recent snowfall has occurred. By observing precipitation from the stations surrounding Sajama volcano and by verifying snow presence on surrounding summits, we detect images with recent snowfall likely to disturb the climatic significance of the snow line. A snow line evolution model takes into account the different image acquisition dates and adjusts the snow line elevation accordingly for the middle of the dry season. A progressive rise of the snow line elevation is observed from 1963 to 1998 with a sustained rise from 1984 to 1990. The snow line altitude is related to the Southern Oscillation Index. Even after the high precipitation of the 1996--1997 wet season, the following El Ni¿o 1997--1998 leads to a substantial rise of the snow line. The snow line elevation is related primarily to the total rainy season precipitation and to a lesser degree to the maximum monthly mean temperature of the warmest month, thus confirming a greater snow line sensitivity to precipitation than to temperature. ¿ 2001 American Geophysical Union