The solar wind conditions of an extreme geomagnetic storm were examined using magnetic field observations obtained from geosynchronous satellites and the disturbance storm-time (Dst) index. During geosynchronous magnetopause crossings (GMCs), magnetic field variations at the magnetosheath, which is the modulated interplanetary magnetic field (IMF), were observed by geosynchronous satellite. The dawn to dusk solar wind electric field (VB_S) was estimated from the Dst index by using an empirical formula for Dst prediction; these data were then used to estimate the IMF and solar wind speed. This method was applied in the analysis of an extreme geomagnetic storm event that occurred on March 13–14, 1989, for which no direct solar wind information was available. A long duration of the GMC was observed after the second storm sudden commencement (SSC) of this event. The solar flare possibly associated with the second SSC of this storm event was identified as the March 12 M7.3/2B flare. The IMF B _z was estimated to be about −50 nT with a solar wind speed of about 960 km/s during the 5 h in which the main phase of the storm rapidly developed, assuming an Alfvén Mach number (M _A) during this period of more than 2.