Quantity and quality of input predicts vocabulary growth (Huttenlocher, Haight, Bryk, Seltzer, & Lyons, 1991; Ramirez-Esparza, García-Sierra, & Kuhl, 2014) and the ability to encode speech quickly and accurately, known as lexical processing efficiency (LPE) (Weisleder & Fernald, 2013). However, little is known about what qualities of speech are strong indicators of language development. Speech style, such as infant-directed speech (IDS), is a special signal for communicating with infants (Fernald, 1985) and may be one quality of input that could impact language outcomes. For example, the exaggerated differences between speech sounds and attention-getting properties of IDS may support LPE; however, little is known about how experience with IDS impacts language development. We tested whether hearing IDS at home predicts vocabulary size and LPE and found evidence of sex-related differences in the relationship between IDS and both vocabulary size and LPE. We collected home-recordings of input in 12 month olds' (N = 38, Males = 19) environment and coded how much IDS they heard. At 12, 15, and 18 months, we collected parent-report measures of receptive and productive vocabulary and assessed infants' ability to recognize common words (e.g. baby, doggy) using lab-based looking-while-listening paradigm. Females who heard more IDS had larger receptive vocabularies at 12 months and 15 months and a hint of word-recognition. In contrast, males showed the opposite effect, such that hearing more adult-directed speech (ADS) predicted larger receptive vocabularies at 12 months and 15 months, but the amount of IDS or ADS they heard did not relate to their LPE skills. This suggests that hearing more IDS may benefit females' early language development, but that hearing ADS may be advantageous for males' receptive vocabulary development.