An additional protocol was added to the 1947 Poland-Czechoslovakia Treaty in which both parties pledged to resolve mutual territorial disputes and provide the Polish people living in Czechoslovakia and the Czechs and Slovaks living in Poland reciprocal minority rights within two years. Poland proposed further prolongation of the additional protocol several months before expiration of the term. Initially, the Czechoslovak party did not favour this change, but finally it agreed with secret prolongation of the protocol. This was achieved by exchange of letters between foreign ministers; both letters were to remain unpublished. The author briefly introduces the researched issue and adds two hitherto unknown documents, which bring further information on the Czechoslovak reaction to the Polish proposal regarding prolongation of the additional protocol and presents copy of the secret letter with which the Czechoslovak government was acquainted at a cabinet meeting.