This study investigated the role of joint attention in infants' word learning. Infants aged 18–21 months were taught new words in two social contexts, joint attention (eye contact, positive tone of voice) or non-joint attention (no eye contact, neutral tone of voice). Event-related potentials were measured as the infants saw objects either congruent or incongruent with the taught words. For both social contexts, an early negativity was observed for the congruent condition, reflecting a phonological-lexical priming effect between objects and the taught words. In addition, for the joint attention, the incongruent condition elicited a late, widely distributed negativity, attributed to semantic integration difficulties. Thus, social cues have an impact on how words are learned and represented in a child's mental lexicon.