In five experiments we examined the way in which readers reanalyze garden-path sentences, using grammaticality judgments as the dependent measure. The stimuli were twoclause sentences containing an ambiguous noun phrase which could function as either the object of the first clause or the subject of the second. Prior research has shown that the former analysis is generally preferred. In the first two experiments, we varied the number of words in the ambiguous phrase and found that reanalysis of garden-path sentences was more difficult with a longer ambiguous phrase. The third experiment established that this effect of phrase length is not attributable to the greater syntactic complexity of longer phrases. The fourth and fifth experiments demonstrated that the effect of phrase length is attributable to increasing the distance from the head of the ambiguous phrase to the disambiguating word of the graden-path sentence: Ambiguous phrases made long by the addition of prenominal adjectives were easy for the parser to reanalyze, but phrases made long by the addition of postnominal modifying prepositional phrases (Experiment 3) or relative clauses (Experiments 4 and 5) were hard for the parser to reanalyze. From these results, we argue that sentence comprehension requires the creation of phrase structure and the assignment of thematic roles to phrases, with the assignment taking place at the phrasal head. Reanalysis is affected by the ease with which thematic roles can be reassigned to misanalyzed phrases.