This paper reports a detailed comparison at the global, hemispheric, three 60°-latitude zone, and grid-box scale between lower tropospheric temperatures from the microwave sounding units (MSU2R) on board NOAA polar orbiting satellites and surface temperatures. The comparisons reveal differences in the course of temperature trends over the 1979-1996 period in the two sets of time series. The surface data warms relative to MSU2R by 0.19°C per decade over this period, with much of the change occurring as a jump in the difference series, particularly during 1991 but also in 1981. The differences either reflect problems in one or both of the surface or MSU2R records or, if both records are correct, a significant change in lapse rates in the lower part of the atmosphere on a global scale particularly since mid-1991. Although MSU2R data agree well with radiosonde data on global and hemispheric scales over 1979-1996, surface and radiosonde data agree almost exactly when their trends are compared over 1958-1993 and 1965-1996. The differences in the two data sets are discussed in terms of possible natural causes and data homogeneities.