A number of studies have documented the features and investigated the causes of the large tropospheric cooling over East Asia since the 1950s seen in both the ERA-40 and NCEP/NCAR reanalyses. This cooling, which is centered around (43degN, 106degE) and peaks at 300hPa and in April and July-August, has also been linked to the weakening in the East Asian summer monsoon during recent decades. This tropospheric cooling is unusual because of its large magnitude (>2degC from 1950-1976 to 1977-2000) and the fact that the troposphere has warmed over most other regions during the same period. Our closer examination revealed that this tropospheric cooling occurred mainly from the 1950s to the early 1970s, when many changes in observational practices occurred at the radiosonde stations around the cooling center. Radiosonde temperature records from these stations, which were assimilated into the reanalysis products without homogenization, show several stepwise drops from the 1950s to the 1970s that are likely caused by changes in observational practices. The collocated reanalysis temperature time series from the reanalyses show temporal evolutions similar to the station series. The 20th century reanalysis (20CR), which was forced only by observed sea surface temperatures and sea-level pressure without the use of radiosonde and satellite data, does not show such a large cooling over East Asia since the 1950s.The radiosonde temperature data after a preliminary homogenization do not show the large cooling. These analyses suggest that the large cooling seen in the reanalysis data may be spurious resulting from errors in the radiosonde data during the earlier decades, and they demonstrate the urgent need to produce homogenized radiosonde data for use in future atmospheric reanalyses.