The first Special Sensor Microwave Imager (SSMI) was launched on the Defence Meteorological Satellite Program (DMSP) F8 spacecraft in July 1987, and wind speed was no longer retrieved after December 1991. A second SSMI was launched on DMSP F10 in December 1990. Interpretation of the 1987--1993 (or longer) SSMI wind speed time series is dependent upon the space and time characteristics of the differences between F8 and F10 SSMI measurements. The 10¿-zonal averaged monthly mean F8-F10 wind speed difference was negative (positive) for wind speeds less (greater) than 7.9 m s-1, reaching -0.43 (0.32) m s-1 at 5 (10) m s-1. Between 60¿S and 60¿N the 10¿-zonal averaged monthly mean F8-F10 wind speed bias was greater than ¿0.5 m s-1 on several occasions. From 60¿S-60¿N the 1991 average value of the monthly mean root-mean-square difference between daily F8 and F10 wind speeds in 10¿-longitudinal bands was 2.0 m s-1. In the 60¿S-60¿N region, about 50% of the daily F8 and F10 wind speed differences was caused by measurement non-simultaneity and about 50% of the difference was attributed to other factors, such as instrument noise and the different azimuthal orientations of each SSMI. Caution is advised in the interpretation of large-scale monthly mean analyses obtained from a combined F8 and F10 wind speed time series.