Solar wind plasma data from ISEE 3 at the L1 point and IMP 8 in Earth orbit are compared to determine how well an L1 monitor predicts plasma conditions at Earth. These data cover the time period August 1978 to February 1980, approaching solar maximum. Data are divided into 6-hour blocks, time shifted to compensate for the radial separation of the spacecraft, and interpolated to provide identically sampled time series; then linear correlation coefficients are calculated as a function of lag. The average correlations of solar wind speed, density, and flux are all about 0.6. The most important factors determining the degree of correlation are the radial separation of the spacecraft and the standard deviation of the density. For the largest standard deviations of density, the correlation coefficients are 0.85. The lag between observation of solar wind features at the two spacecraft varies with both the radial and azimuthal separation of the spacecraft, indicating that both radial propagation of the solar wind and the rotation of the Sun are important effects for determining solar wind arrival time. Plasma structures seem to be less well correlated than magnetic field structures. ¿ 1998 American Geophysical Union