Purpose: To determine whether a clinically obtainable measure of audibility, the aided Speech Intelligibility Index (SII; American National Standards Institute, 2007), is more sensitive than the pure-tone average (PTA) at predicting the lexical abilities of children who wear hearing aids (CHA). Method: School-age CHA and age-matched children with normal hearing (CNH) repeated words and nonwords, learned novel words, and completed a standardized receptive vocabulary test. Analyses of covariance allowed comparison of the 2 groups. For CHA, regression analyses determined whether SII held predictive value over and beyond PTA. Results: CHA demonstrated poorer performance than CNH on tests of word and nonword repetition and receptive vocabulary. Groups did not differ on word learning. Aided SII was a stronger predictor of word and nonword repetition and receptive vocabulary than PTA. After accounting for PTA, aided SII remained a significant predictor of nonword repetition and receptive vocabulary. Conclusions: Despite wearing hearing aids, CHA performed more poorly on 3 of 4 lexical measures. Individual differences among CHA were predicted by aided SII. Unlike PTA, aided SII incorporates hearing aid amplification characteristics and speech-frequency weightings and may provide a more valid estimate of the child's access to and ability to learn from auditory input in real-world environments.