This paper discusses a colloquial variety of Polish relative clauses introduced by the uninflected relative marker co. Unlike previous accounts, the analysis concentrates on authentic spoken utterances marked by structural unintegration—a common feature of spontaneous spoken language. As is shown, co clauses in unplanned speech depart from the traditional perception of what function they perform and how they do it. The advantage of using corpus data is that they offer insight into a wider range of functions of co than previously reported. These functions include a weakly subordinating conjunction, a general discourse connective, and time- and place-reference conjunctions similar to English when and where. Additionally, some cases are ambiguous as to which of these functions co serves. The basic relativizing use of co is also revised and its description is enriched by an analysis of co clauses in spontaneous speech, in which several unintegration features were observed. They are in general related to the loose syntactic relationship of the head NP to the co clause. Specific features of unintegration include (i) co clauses as complete clauses with no gaps, (ii) idiosyncrasy and context-dependency of interpretation, (iii) nonmatching case forms and lack of required resumptive pronouns, (iv) preposition ellipsis, (v) long-distance relationship between the head and co clause, (vi) ambiguity in the semantic contribution of co clauses and of the marker co itself, and (vii) lack of a clearly specified nominal head.