Halo coronal mass ejections (HCMEs) originating from regions close to the center of the Sun are likely to be geoeffective. Assuming that the shape of HCMEs is a cone and that the HCMEs propagate with constant angular widths and velocities, at least in their early phase, we have developed a technique which allowed us to obtain the space speed, width, and source location. We apply this technique to obtain the parameters of all full HCMEs observed by the Solar and Heliospheric Observatory (SOHO) mission's Large Angle and Spectrometric Coronagraph (LASCO) experiment until the end of 2002. Using this data, we examine which parameters determine the geoeffectiveness of HCMEs. We show that in the considered period of time, only fast halo CMEs (with space velocities higher than ∼1000 km/s) and originating from the Western Hemisphere close to the solar center could cause intense geomagnetic storms. We illustrate how the HCME parameters can be used for space weather forecast. It is also demonstrated that the strength of a geomagnetic storm does not depend on the determined width of HCMEs. This means that HCMEs do not have to be very large to cause major geomagnetic storms.