This study attempts to determine the common features and differences between the Latin language of the inscriptions of Aquincum, Salona, Aquileia and the provincial countries of Pannonia Inferior, Dalmatia and Venetia et Histria, compared with each other and the rest of the Latin speaking provinces of the Roman empire, and we intend to demonstrate whether a regional dialect area over the Alps–Danube–Adria region of the Roman empire existed, a hypothesis suggested by József Herman. For our research, we use all relevant linguistic data from the Computerized Historical Linguistic Database of Latin Inscriptions of the Imperial Age. We will examine the relative distribution of diverse types of non-standard data found in the inscriptions, contrasting the linguistic phenomena of an earlier period with a later stage of Vulgar Latin. The focus of our analysis will be on the changes in the vowel system and the grammatical cases between the two chronological periods within each of the three examined cities. If we succeed in identifying similar tendencies in the Vulgar Latin of these three cities, the shared linguistic phenomena may suggest the existence of a regional variant of Latin in the Alps–Danube–Adria region.