Winter and summer mass balance measurements from four French glaciers have been used to assess the sensitivity of mass balance to climatic fluctuations. The sensitivity of summer ablation to temperature is maximum in low-elevation zones (1.4 m water equivalent (w.e.) ¿C-1 at 1800 m above sea level (asl)) and decreases with altitude (0.5 m w.e. ¿C-1 at 2900 m asl). As a consequence, the sensitivity of equilibrium line altitude to temperature is 60--70 m ¿C-1. This is half the value previously reported in the literature, implying that alpine glacier retreat scenarios for the 21st Century have been largely overestimated. Winter accumulation can be as high as 3 times the amount of precipitation recorded downvalley. These relationships between mass balance and meteorological data were then used to reconstruct the mass balances of these four glaciers back to 1907 using old maps and photogrammetric measurements. Model sensitivity analysis shows that a 25--30% increase in precipitation would compensate a 1¿C temperature rise for the mass balances of glaciers. From these results the 20th Century may be divided into four periods: two steady state periods, 1907--1941 and 1954--1981, during which the mass of glaciers remained almost constant, and two deficit periods, 1942--1953 and 1982--1999, marked by a sharp reduction in glacier mass. Regarding mean ablation at 2800 m asl, a 22 W m-2 increase in energy balance is required to explain the ablation difference between the two most recent periods, 1954--1981 and 1982--1999. According to the energy balance analysis the increase in air temperature explains more than 60% of this ablation rise.