In a prospective study of child development in relation to early-life otitis media, we administered the MacArthur Communicative Development Inventories (CDI) to a large (N = 2,156), sociodemographically diverse sample of 1- and 2-year-old children. As a prerequisite for interpreting the CDI scores, we studied selected measurement properties of the inventories. Scores on the CDI/Words and Gestures (CDI-WG), designed for children 8 to 16 months old, and on the CDI/Words and Sentences (CDI-WS), designed for children 16 to 30 months old, increased significantly with months of age. On several scales of both CDI-WG and CDI-WS, standard deviations approximated or exceeded mean values, reflecting wide variability in results. Statistically significant differences in mean scores were found according to race, maternal education, and health insurance status as an indirect measure of income, but the directionality of differences was not consistent across inventories or across scales of the CDI-WS. Correlations between CDI-WG and CDI-WS ranged from .18 to .39. Our findings suggest that the CDI reflects the progress of language development within the age range 10 to 27 months. However, researchers and clinicians should exercise caution in using results of the CDI to identify individual children at risk for language deficits, to compare groups of children with different sociodemographic profiles, or to evaluate the effects of interventions.