The first quantitative inference of the past solar wind conditions is successfully attempted. Using Godhavn magnetometer data, we are able to deduce the gross temporal variations of the solar wind back to 1926. First, a set of daily perturbation quantities is defined from the hourly magnetic elements H, D, and Z characterizing different aspects of the high-latitude current system. An extensive analysis by means of robust multiregression methods enable us for the satellite era to chart the sensitivity of the perturbation quantities to specific interplanetary parameters. Second, at an arbitrary day the derived statistical relations are used to invert the geomagnetic response to give the daily averages of the southward interplanetary magnetic field (IMF) component Bs, the azimuthal IMF component By, and the solar wind speed V. It is necessary to incorporate a priori observations of Bs, By, and V in the least squares solution but only with very low weights relative to those of the Godhavn observations. The inversion procedure is verified and optimized on solar cycle 20 and 21. Reconstruction results are given for solar cycle 16, 17, and 18 covering the interesting period with a steep long-term increase of the solar activity. The cyclic features observed in situ are also found for the deduced solar wind conditions: a pronounced modulation of Bs and By, and the maximum in V occurring a few years before the activity minimum. A systematic trend is only observed for the solar wind speed. This occurs in the first half of cycle 16 and cycle 17, where the normal variations appear superposed on a linear increase. A distinct increase in the solar wind speed is apparent through cycle 17.¿ 1997 American Geophysical Union