The article examines the role of infant-directed vocabulary (IDV) in infants language acquisition, specifically addressing the question of whether IDV forms that are not prominent in adult language may nonetheless be useful to the process of acquisition. Japanese IDV offers a good test case, as IDV characteristically takes a bisyllabic H(eavy)-L(ight) form that is rare in adult speech. In 5 experiments using the Headturn Preference Procedure (HPP), 8- to 10-month-old Japanese infants, but not 4- to 6-month-olds, were found to show a preference for bisyllabic H-L words over other types of words. These results demonstrate (a) that infants may develop a preference for a dominant prosodic form based on infant-directed speech, even when it is not a prominent characteristic of adult language; and perhaps more importantly, and (b) that infant-directed speech may provide a boost for a feature that could be useful for infants' acquisition of language even when it not prominent in adult language.