A time-dependent cylindrically symmetric computer model of the Hofsjökull ice cap, Central Iceland, has been defined in order to investigate the time-scale of possible glacier variations. The model is forced with a set of hypothetical mass balance scenarios, chosen to span the range of plausible mass balance variations of Hofsjökull caused by greenhouse warming. For each mass balance scenario the model computes the change in the volume of the ice cap and the associated time-dependent runoff variations. The response time of the ice cap is found to be of the order of 50-100 years,˜when mass balance - elevation feedback is taken into account. The response time increases with the size of the climatic perturbation. The model indicates that increased runoff from the area now covered by Hofsjökull, caused by a possible greenhouse warming, will be significant for 100-200 years after the climate starts to change. A mass balance scenario produced by raising the equilibrium line by 200 m (corresponding roughly to a warming of 1-1.5°C) over a 50-year period leads to a maximum runoff increase of about 40 m3s−1 occurring 50 years after the equilibrium line starts to rise. This amounts to approximately 50 % of the total precipitation that falls on Hofsjökull at present and approximately 25 % of the present discharge of rivers that issue from Hofsjökull and the neighbouring area.