By comparing the recorded run-off from a precipitation district, about 55 per cent of which is covered by glaciers, to the run-off from a neighbouring district without glaciers it has been possible to determine the changes in the glacier mass of Jostedalsbreen. In the period 1900 to 1940 the annual decrease of the glacier mass was equivalent to 0.4 m. of water times the glaciated area. A close relationship is established between the change of the glacier mass and the length of one of the outflow glaciers, Brikadalsbreen. It is shown that the change in the glacier mass is related to the summer temperature and, to a minor extent, to the amount of winter snowfall.