Ice-capped volcanoes of the Chilean Lake District have shown significant glacier retreat during recent decades, probably in response to tropospheric warming and decrease of precipitations. Volcán Mocho-Choshuenco (39º55’S, 72º02’W) is one of the main active volcanoes of this part of the country where a mass balance programme was initiated on its south-eastern glacier in 2003, in view of its representative conditions as an ice body presumably not affected by current volcanic activity. The glaciers of this volcano have been retreating and shrinking in recent decades, totalizing in 2003 a reduction of 40% of the original area of 1976 (28.4 km²). A maximum area change was obtained in the most recent analysed period, when a rate of -0.45 km² yr^-1 between 1987 and 2003 was obtained. The glacier average net mass balance of 2003/2004 yielded -0.88 m ± 0.09 m of w. e. yr^-1, with an average net accumulation and ablation of 2.59 and -3.47 m of w. e. yr^-1, respectively. This is the first direct measurement of glacier mass balance in southern Chile, where very little is known about glacier variations and glacier volcano interactions.