The partial melting of natural lherzolite KLB-1 at 1 GPa was studied for both H2O undersaturated and saturated conditions. The quenched glasses obtained at 1000 and 1050 °C are andesitic in composition, having 54.4–60.3 wt% SiO2, whereas partial melts formed at temperatures above 1100 °C have basaltic compositions. This suggests that high-magnesian andesites can be formed by direct partial melting of the hydrous upper mantle only at relatively low temperatures. These andesitic melts, together with those obtained at 1100 °C, contain more than 3 wt% H2O and have compositions characteristic of hydrous melts—high SiO2 and Al2O3 and low FeO*, MgO, and CaO. The results of melting studies on KLB-1 show a strong increase in the degree of melting when H2O is added to lherzolite at a given temperature.