The Low Level Jet East of the Andes (LLJ) is an integral component of the South American Monsoon System. Currently, the scarce upper-air observational network does not allow having a clear picture of the spatial structure and time variability of this circulation feature. The availability of few pilot balloon observations in Bolivia since 1998 has helped in identify some LLJ events and relate them to convective development and rainfall in Southeastern South America. However, there are several characteristics of the LLJ that still remain little known. Therefore, we develop climatology of circulation and moisture related to LLJ episodes using the NCEP reanalyses during 1950-2000, and make a validation of LLJ episodes identified from pilot balloon observations made in Bolivia and Paraguay in 1998-2000. Upper and low-level circulation fields relevant to the presence of the LLJ were determined for mean summer and wintertime, and for composites of LLJ events during those seasons, and we also investigate the diurnal, seasonal, annual and interannual variability of LLJ episodes. Some preliminary results show that LLJ seems to occur all year long, with the LLJ bringing tropical air masses from the Amazon into southern Brazil-Northern Argentina more frequent in summer, and the LLJ that brings tropical maritime air from the Subtropical Atlantic High more frequent during winter. The diurnal cycle shows that LLJs are more frequent and intense between 06 and 12 (01 and 07 LST) for the warm season in the core region of the jet, while at the exit region the maximum is detected between 00 and 06 Z. during the cold season. LLJ episodes show an upper level circulation a wave train emanating from the West Pacific propagating towards South America, and the intensification of the LLJ obeys to an establishment of an upper-level ridge over southern Brazil and a trough over most of Argentina.