Children, 4-6 years of age, in special education kindergartens were randomly assigned to a classification training (n 45) and a comparison (n 49) group. Children in the training group were taught the Classification unit of Bright Start, whereas those in the comparison group received a regular content-oriented curriculum. Both groups were given pre- and post-tests of classification, semantic categories, and conceptual and perceptual analogies. Children who received the classification training improved more on all tests than did those in the comparison group. Significant positive correlations were found between verbal conceptualization and classification, conceptual analogies, and perceptual analogies. Teaching classification appears to have effects that generalize to other domains of language and higher order thinking that are significant in the cognitive development of young children with developmental language delays. The findings support the interplay between thinking and language and positive cognitive developmental effects of training in classification.