Until recently, the challenges inherent in measuring comprehension have impeded our ability to predict the course of language acquisition. The present research reports on a longitudinal assessment of the convergent and predictive validity of the MacArthur-Bates Communicative Development Inventories: Words and Gestures (CDI: WG; Fenson et al., 1994, 1993, 2007) and the Computerized Comprehension Task (CCT; Friend & Keplinger, 2003, 2008). The CDI: WG and the CCT evinced good convergent validity; however, the CCT better predicted subsequent parent reports of language production. Language sample data in the 3rd year confirm this finding: The CCT accounted for 24% of the variance in unique word use. These studies provide evidence for the utility of a behavior-based approach to predicting the course of language acquisition into production.