The subject of the article is the flexion of the Polish homonymous names in the masculine. The term “homonyms” means the names of different objects that have the same shape. There are homonyms that have the same form in the nominative, but belong to different paradigms. The homonyms are segregated from the diachronic point of view, considering the sequence of linguistic phenomena over time. Just this episode contributes to the emergence of alternative forms. The homonyms appearance of flexion is rather common, as the homonymyty often occur in the Polish onomastics. For both place names as well as under ordinary nouns in the genitive singular, there are alternative forms. The genitive of homonyms in the masculine singular has two endings: -a and -u. There are three possibilities: the ending -a, the ending -u or alternatively: -a or -u. Regarding the lack of clear criteria we often face the problem of choosing the appropriate ending, for example: Frombork, Fromborka; Lidzbark, Lidzbarka, but Rombark, Rombarku; Tarnobrzeg, Tarnobrzega, but also Tarnobrzegu. Most problems do not just alternative forms for example Borek, Borka//Borku, Turek, Turku//Turka... etc. Toponyms which accept the genitive the ending -a, for example: Aleksandrów, - a; Baranów, -a; Borowiec, -a; Chełmek, -a; Grabnik, -a; Jarocin, -a; Łąck, -a; Zagórz, -a. Nouns ending in -u in the genitive there are substantially less and only a few can be seen as typical of the names with -u, for example: Czarnolas, -u; Gózd, -u; Łęg, -u; Międzychód, -u; Obrąb, -u; Staw, -u. The types of roots endings that take in both the genitive ending -a and -u, cause the biggest problem. They are the names: Brzostek, -a, but also -u; Dębniak, -a // -u; Dworek, -a // -u; Poraj, -a // -u; Szadek, -u // -a; Tarnobrzeg, -a // -u; Turek, -u // -a; Wiatrak, -a // -u; Zalas, -u, but also -a. Usually in most names in the genitive ending occurs -a. Only a small percentage of all names in the masculine gets the ending -u, and among them there is a clear tendency to replaced with the forms ending -u by forms with the ending -a. In the case of homonymous names facilitates not the formal criterion of the user to choose a correct form of the dependent case, particularly in the genitive the formal criterion does not make it easier for the user. In terms of the identical macro and microtoponyms, one should make the choice not only on the basis of knowledge of the flexion of toponyms and appelatives, but one should also know the regional determinants. Udostępnienie publikacji Wydawnictwa Uniwersytetu Łódzkiego finansowane w ramach projektu „Doskonałość naukowa kluczem do doskonałości kształcenia”. Projekt realizowany jest ze środków Europejskiego Funduszu Społecznego w ramach Programu Operacyjnego Wiedza Edukacja Rozwój; nr umowy: POWER.03.05.00-00-Z092/17-00.