Fifty years of monthly mean sea level pressure (MSLP) data from the NCEP/NCAR reanalysis (hereinafter NNR) are validated against station observations in the extra-tropical South Pacific and West Antarctica. The period for which the NNR may be considered reliable differs markedly between regions. It is only realistic from ~1970 across West Antarctica. A lack of surface observations entering the model causes the NNR to be poorly constrained prior to the availability of GTS-based data in 1967: the latter coincides with a significant improvement in the NNR MSLP across the eastern South Pacific. Substantial spurious negative trends occur within the NNR MSLP at high latitudes; although the decrease is greatest prior to the advent of satellite sounder data it continues into the 1990s south of 60 ¿S. In addition, widely used tropical and extra-tropical circulation indices are poorly represented in the NNR data prior to the 1960s. ¿ 2000 American Geophysical Union