Infant-directed speech (IDS), compared with adult-directed speech (ADS), is characterized by a slower rate, a higher fundamental frequency, greater pitch variations, longer pauses, repetitive into-national structures, and shorter sentences. Despite studies on the properties of IDS, there is no direct demonstration of its effects for word learning in infants. This study examined whether 21- and 27-month-old children learned novel words better in IDS than in ADS. Two major findings emerged. First, 21-month-olds reliably learned words only in the IDS condition, although children with relatively larger vocabulary than their peers learned in the ADS condition as well. Second, 27-month-olds reliably learned the words in the ADS condition. These results support the implicitly held assumption that IDS does in fact facilitate word mapping at the start of lexical acquisition and that its influence wanes as language development proceeds.