In two experiments participants read words and pseudo-words that belonged to either large or small lexical neighborhoods while event-related brain potentials (ERPs) were recorded from their scalps. In Experiment 1, participants made speeded lexical decisions to all items, while in Experiment 2 they engaged in a go/no-go semantic categorization task in which the critical items did not require an overt behavioral response. In both experiments, words and pseudo-words produced a consistent pattern of ERP effects: items with many lexical neighbors (large neighborhoods) generated larger N400s than similar items with relatively fewer lexical neighbors (small neighborhoods). Reaction time (RT, Experiment 1), on the other hand, showed a different pattern consistent with previous behavioral studies. While words tended to produce a facilitation in RT for larger neighborhoods, pseudowords produced an inhibition effect. The findings are discussed in terms of recent theories of word recognition and the functional significance of the N400.