Measurements of mass balance were performed every month on Zongo Glacier. Bolivia. Simultaneously, water-discharge, temperature and precipitation data were obtained. The first year of the survey, 1991–92. was marked by an ENSO (El Niño-Southern Oscillation) event with high temperature and low precipitation, whilst the following year, 1992–93, was normal. Results point to the early and late wet season (October-December and March–May: as playing a critical role in the determination of the annual mass balance. The wet season is the warmest period of the year and consequently the duration of the wet season is a highly relevant variable in determining mass balance. Both glaciological and hydrological methods for the determination of the mass balance provide similar results. Our study confirms dial ENSO events have a major influence on the rapid glacier retreat currently affecting this part of the Andes.