Children who rapidly recognize and interpret familiar words typically have accelerated lexical growth, providing indirect evidence that lexical processing efficiency (LPE) is related to word-learning ability. Here we directly tested whether children with better LPE are better able to learn novel words. In Experiment 1, 17- and 30-month-olds were tested on an LPE task and on a simple word-learning task. The 17-month-olds' LPE scores predicted word learning in a regression model, and only those with relatively good LPE showed evidence of learning. The 30-month-olds learned novel words quite well regardless of LPE, but in a more difficult word-learning task (Experiment 2), their LPE predicted word-learning ability. These findings suggest that LPE supports word-learning processes, especially when learning is difficult. Children who rapidly recognize and interpret familiar words typically have accelerated lexical growth, providing indirect evidence that lexical processing efficiency (LPE) is related to word-learning ability. Here we directly tested whether children with better LPE are better able to learn novel words. Our results suggest that LPE supports word-learning processes, especially when learning is difficult.