Syntactically ambiguous sentences have been found to be difficult to process, in particular, for individuals with low working memory capacity. The current study used fMRI to investigate the neural basis of this effect in the processing of written sentences. Participants with high and low working memory capacity read sentences with either a short or long region of temporary syntactic ambiguity while being scanned. A distributed left-dominant network in the peri-sylvian region was identified to support sentence processing in the critical region of the sentence. Within this network, only the superior portion of Broca's area (BA 44) and a parietal region showed an activation increase as a function of the length of the syntactically ambiguous region in the sentence. Furthermore, it was only the BA 44 region that exhibited an interaction of working memory span, length of the syntactic ambiguity, and sentence complexity. In this area, the activation increase for syntactically more complex sentences became only significant under longer regions of ambiguity, and for low span readers only. This finding suggests that neural activity in BA 44 increases during sentence comprehension when processing demands increase, be it due to syntactic processing demands or by an interaction with the individually available working memory capacity.