In this paper, we discuss the solar origin and interplanetary consequences of the coronal mass ejection of March 29, 2001 that was responsible for the most intense geomagnetic storm (DST ~ -377 nT) of the current solar cycle to date. A comparison of the CME of March 29, 2001, with a set of geo-effective halo CMEs associated with X-class flares showed that the strength of the geomagnetic storm at the earth is well correlated with the speed of the halo. Our study shows that the fast ejection is responsible for building up the ram pressure at the earth's magnetosphere. This may serve as a useful tool in the forecasting of intense geomagnetic storms.