This paper discusses the problem of resumptive pronouns in Polish object relative clauses introduced by the relative marker co. It does so through the use of corpus data, thus contributing to previous literature, which has been largely based on introspection. In the literature, different accounts vary significantly as to the basic question of when the resumptive pronoun is expected. The present study addresses this matter by means of qualitative and quantitative analysis of conversational spoken Polish—the language variety in which co relatives typically occur. As is shown, the relatives are used in two broad configurations—unmarked (with null resumptives and inanimate referents) and marked (with overt resumptives and human referents). Both scenarios are linked to distinct strategies of case recovery. The presence of the pronoun itself is one such strategy. In contrast, the omission of the pronoun is often accompanied by case-matching effects that facilitate the omission. Another typical property of co relatives is their preference for encoding definiteness of referents, whereby który clauses tend to signal indefiniteness. This is evidenced by the frequent cooccurrence of co clauses with head-internal demonstratives. Interestingly, these head-internal demonstratives can also render resumptive pronouns unnecessary, thus constituting another factor relevant in resumption.