In adults, a salient tone embedded in a sequence of nonsalient tones improves detection of a synchronously and briefly presented visual target in a rapid, visually distracting sequence. This phenomenon indicates that perception from one sensory modality can be influenced by another one even when the latter modality provides no information about the judged property itself. However, no study has revealed the age-related development of this kind of cross-modal enhancement. Here we tested the effect of concurrent and unique sounds on detection of illusory contours during infancy. We used a preferential looking technique to investigate whether audio-visual enhancement of the detection of illusory contours could be observed at 5, 6, and 7 months of age. A significant enhancement, induced by sound, of the preference for illusory contours was observed only in the 7-month-olds. These results suggest that audio-visual enhancement in visual target detection emerges at 7 months of age.