In this article, a conversation between Old Helsinki Slang (OHS) speakers recorded in 1965 is examined. A notable feature of OHS is the heavy use of Swedish-based or otherwise un-Finnish words although it mostly follows the grammar of colloquial Finnish. e sample that is analyzed consists of free speech, and it lasts 65 minutes. If uncertain items are taken into account, then the proportion of borrowed lexical items in the data is 29–32%. Function and content words in OHS di er markedly in their etymological origin as the function words are overwhelmingly Finnish. Although OHS has some phonological and phonotactical features that are strikingly “un-Finnish,” it is apparent that these features have been adopted along with loanwords. While some morpho-syntactical features in OHS di er from those of Standard Finnish, they are widely known in Finnish dialects and colloquial Finnish and, therefore, cannot be interpreted as innovations in OHS. Morpho-syntactically, the sample can easily be interpreted as a variant of Finnish. While the proportion of borrowed words in OHS is not exceptional among the world’s languages, it is in any case notable; furthermore, core borrowing is common and even basic vocabulary is the product of borrowing. Roughly 40% of the vocabulary of OHS can be de ned as slang, a proportion unknown in Finnish dialects or in Standard Finnish. is slang vocabulary is overwhelmingly borrowed, and it can be seen as the most apparent contact feature of OHS. It has made this variety of urban speech virtually incomprehensible to contemporary dialectal or Standard Finnish speakers.