A survey of IPS, geomagnetic and interplanetary medium data has been used to interpret the weak correlation between IPS data and geomagnetic activity. Our results indicate that: (i) we can use IPS data to predict the maximum possible level of geomagnetic activity, given the state of the solar wind; but (ii) to predict the actual level of activity we also require an estimate of the coupling between the solar wind and the magnetosphere. This is the first time that the potential (and the limitations) of IPS for predicting geomagnetic activity have been demonstrated quantitatively in a comprehensive survey. ¿ American Geophysical Union 1994