Upper tropospheric water vapor measurements from the UARS Microwave Limb Sounder are used to investigate the structure and evolution of eastward traveling medium-scale wave features in Southern Hemisphere summertime. The extratropical Southern Hemisphere summer circulation pattern is frequently dominated by medium scale waves which exhibit life cycles of baroclinic growth and barotropic decay. The water vapor field during such life cycles is examined here and found to be well correlated with meteorological fields derived from European Centre for Medium Range Weather Forecasts global analyses. From mid January to mid February 1992, several episodes of growth and decay in the amplitude of eastward traveling waves are found in the water vapor and meteorological data at levels of the upper troposphere, with zonal waves four, five and six being predominant modes. The water vapor data are compared with derived potential vorticity (PV) fields with strong anticorrelation observed in middle and high latitudes. The results are consistent with model paradigms for the structure and evolution of baroclinic disturbances, coupled with the known characteristics of high PV and low water vapor mixing ratios in lower stratospheric air parcels and the reverse for upper tropospheric air. ¿ American Geophysical Union 1996