In the context of language processing, the N280 is an anterior negative event-related potential profile associated with the lexical categorization of grammatical function words versus content words. Subsequent studies suggested that this effect was related to word statistics including length and frequency in the lexicon. The current research tests the hypothesis that the N280 effect is related to an index of grammatical complexity. We recorded event-related potentials during a sentence reading task. Comparing content versus function words revealed the classic N280. Within function words, we compared the relative pronouns 'qui' and 'que' (which are identical for length and frequency) that in French indicate a subsequent simple (subject-subject) and complex (subject-object) relative clause, respectively. A left anterior N280 effect was observed only for 'que', supporting our hypothesis that the N280 reflects grammatical complexity that can be confounded with lexical category and statistical properties.