The author presents the results of recent research carried out at the Department of Old Polish, of the Institute of Polish Laguage of the Polish Academy of Sciences, into the 14th and 15th century lexicons (letters A through C). Its results manifest an urgent need for a new edition of the 'Slownik staropolski' (Old Polish Dictionary). The lexical items collected by the author were not included in the first edition of the dictionary, as the sources were not known at the time of its printing in 1953. Since then, over ninety medieval manuscripts have come out in print or have been reedited. The wealth of data is illustrated in the article with the examples of 'bialoskornik', 'bezprawy', 'czeczot', and 'czynca', and they should be of interest to a wider linguistic community. The article also describes the methods of searching for new lexical items, isolating them, and accounting for their meanings.