Research on immediate effects of background television during mother-infant toy play shows that an operating television in the room disrupts maternal communicative behaviors crucial for infants' vocabulary acquisition. This study is the first to examine associations between frequent background TV/video exposure during mother-infant toy play at home and subsequent maternal speech characteristics and infant vocabularies. Twenty-five mothers completed a survey of background television exposure and a vocabulary measure for infants aged 13 and 17 months. Mothers' total word production and numbers of different words at each age were calculated from transcripts of play interactions with no television present. Greater exposure to background television during dyadic play "negatively" predicted infants' vocabularies and mothers' speech quantity and lexical diversity at 17 months. Moreover, these maternal speech characteristics fully mediated the association between exposure to background television during dyadic play and infants' expressive vocabulary acquisition.