We know about a significant number of inscriptions – the major part of them were found in Rome - in which the pronoun idem, the form of the nominative masculine, stands in the place of another grammatical gender or case of the same word (usually a dative), or in the place of the adverb item. In the edited epigraphic corpora, this form is usually interpreted as adverbial and emendated for item. However, in similar context (as for example in the title), we can often see isdem too, the archaic form of the nominative masculine, which cannot be explained on the base of the phonology as derivated from item. In the 19th century, Friedrich Ritschl thought that these forms substituted in reality eidem (dative singular of idem), and explained the change based on phonology (eidem to idem), and then on analogy (idem to isdem). An explanation like this imply the fossilisation of the pronoun, since the variants of the nominative masculine occure in the place of another inflected form of the word, specifically in the dative. In 1907, E. H. Sturtevant published an article (Some Unfamiliar Uses of Idem and Isdem in Latin Inscriptions) in which he intended to refute Ritschl’s claim and to give another interesting interpretation. In his opinion, the fenomenon has different origins in Ostia and in Rome. In his theory, the occurrences of the form idem in a position, which is different from the nominative masculine of the pronoun, are dialectic variants of item if they are from Ostia; though the same forms registrated in Rome are interpreted as consciously used nominatives. In consequence, the fossilisation of the word would be a non-existent fenomenon. The aim of this study is to examine critically Sturtevant’s argumentation concerning the fossilisation of the pronoun idem and its eventual fusion with the adverb item.