This paper deals with short-term forecasting of market shares using market share models. The predictive power of market share models is a subject that has received considerable attention in marketing literature. However, hardly any attention has been paid to the question of how the values of the marketing instruments of competitors can be predicted. This is remarkable since these values constitute the input variables for the market share model. In this paper we will investigate the sensitivity of predicted market shares to different assumptions with respect to competitive behaviour.