This experiment shows that for an English speaker reading aloud, the topic structure of the text affects the amount of lengthening in sentence-final words. The speaker lengthened words less at the end of sentences that were followed by another sentence elaborating on the topic of the first, than at the end of sentences where the subsequent sentence added new information or switched topics. These results show that speech durations are affected by larger-scale linguistic organization, in addition to the well-known effects of local and phrasal structure. Modeling variation at the text or discourse level could improve the comprehensibility of longer passages of synthesized text.