The integration of knowledge during reading was tested in 1,109 secondary school students. Reading times for the second sentence in a pair (Jane's headache went away) were compared in conditions where the first sentence was either causally or temporally related to the first sentence (Jane took an aspirin vs. Jane looked for an aspirin). Mixed-effects explanatory item response models revealed that at higher comprehension levels, sentences were read more quickly in the causal condition. There were no condition-related reading time differences at lower comprehension levels. This interaction held with comprehension- and inference-related factors (working memory, word and world knowledge, and word reading efficiency) in the models. Less skilled comprehenders have difficulty in knowledge-text integration processes that facilitate sentence processing during reading.