Two experiments are described which explore the relationship between parental reports of infants' receptive vocabularies at 1 ; 6 () or 1 ; 3, 1 ; 6 and 1 ; 9 () and the comprehension infants demonstrated in a preferential looking task. The instrument used was the Oxford CDI, a British English adaptation of the MacArthur-Bates CDI (Words & Gestures). Infants were shown pairs of images of familiar objects, either both name-known or both name-unknown according to their parent's responses on the CDI. At all ages, and on both name-known and name-unknown trials, preference for the target image increased significantly from baseline when infants heard the target's label. This discrepancy suggests that parental report underestimates infants' word knowledge. [Research for this article was supported by the British Academy. Portions of the data were presented at the conferences of the International Society for Infant Studies (1999) and the Society for Research in Child Development (2005).]