The P600 component in Event Related Potential research has been hypothesised to be associated with syntactic reanalysis processes. We, however, propose that the P600 is not restricted to reanalysis processes, but reflects difficulty with syntactic integration processes in general. First we discuss this integration hypothesis in terms of a sentence processing model proposed elsewhere. Next, in Experiment 1, we show that the P600 is elicited in grammatical, non-garden path sentences in which integration is more difficult (i.e., ''who'' questions) relative to a control sentence (''whether'' questions). This effect is replicated in Experiment 2. Furthermore, we directly compare the effect of difficult integration in grammatical sentences to the effect of agreement violations. The results suggest that the positivity elicited in ''who'' questions and the P600-effect elicited by agreement violations have partly overlapping neural generators. This supports the hypothesis that similar cognitive processes, i.e., integration, are involved in both first pass analysis of ''who'' questions and dealing with ungrammaticalities (reanalysis).