This paper addresses the issue of the historical development of for fear (that) in English – a prepositional subordinator ushering in fi nite clauses of purpose in which negation is inherently coded, i.e. the content of the subordinate clause is negated by the complementiser which does not contain a negative particle in itself. The rise of this construction is studied within the theory of grammaticalization and it turns out to be a regular case of grammaticalization following the mechanisms of grammaticalization such as desemanticisation, extension and decategorialisation.