The aim of the paper is to present the diachronic changes taking place in legal languages and discuss whether the translators, who for some reason use as an equivalent an obsolete term, may produce a target text which is communicatively ineffective. The research methods applied encompass: the parametric approach to the interlingual comparison of legal terminology for translation purposes, the analysis of pertinent literature on translation and translation errors, the analysis of comparable texts for the purpose of observing diachronic changes (...) in terminology. The diachronic change in terminology most frequently is the result of the amendments of legislation or repealing one act and replacing it with a new one. The terminological changes are especially frequent when the change in the political system occurs and the need appears to adjust the legal system to the new political ideology and principles. For that reason, the authors have analysed in detail three Polish acts on copyright enacted under three different political regimes that is to say (i) after the First World War when Poland regained independence after 123 years of occupation (Act of 29 March 1926 on Copyrights [Ustawa z dnia 29 marca 1926 r. o prawie autorskiem]), (ii) under the communist rule in Poland that lasted from 1945 until 1989 (Act of 10 July 1952 on Copyrights [Ustawa z dnia 10 lipca 1952 r. o prawie autorskim]), and finally (iii) after the collapse of communism in Poland and the period of transition into the democratic and free market society (Act on Copyrights and Related Rights of 4 February 1994 [Ustawa o prawie autorskim i prawach pokrewnych z dnia 4 lutego 1994 roku]). Apart from the main Polish corpus of above-mentioned acts, the authors have also provided examples from some other languages to confirm that the diachronic changes take place everywhere and miscommunication problems resulting from using an obsolete term in translation may occur in any pair of languages. In the last part of the paper the problem of the scale of miscommunication resulting from the usage of obsolete terminology in the target text is discussed. (shrink)