- This study investigated the validity of a parent report measure of vocabulary development, the MacArthur Communicative Development Inventory: Words and Sentences (CDI), in children with and without developmental disabilities. Concurrent validity was examined by comparing results from the CDI and laboratory measures of vocabulary in 44 children with Down syndrome and 46 typically developing children with mental ages from 12 to 27 months. Significant correlations between .70 and .82 were obtained. Predictive validity was examined by measuring the vocabulary of 20 children with Down syndrome and 23 typically developing children first at approximately 20 months mental age and later at a mental age of approximately 28 months. Significant correlations were obtained between the CDI at Time A and all but one of the vocabulary measures at Time B (r = .46 to .66). These results establish the validity of parent measures of vocabulary development for children with Down syndrome and confirm their validity for typically developing children.