The article is devoted to more comprehensive use of medieval onymic resources in research on the history of the Polish language. These materials were used in research on the phonetic development of the Polish language in its earliest period. To date, they have rarely appeared in lexical studies. The body of the oldest appellatives, reconstructed on the basis of proper names, would be a kind of lexicon (supplement) enriching and verifying old Polish lexical material, certifi ed in historical Polish dictionaries (also in etymological dictionaries). In this way, the expectations formulated over 100 years ago by eminent Polish linguists may be fulfi lled. The complementary use of such a huge wealth of material opens up further research perspectives towards etymological, dialectological, lexical and morphological research.