The study summarized in this paper deals with the grammatical analysis of the spontaneous speech of approximately 150 children who are classified as mentally disabled; educable (I.Q. range 50-80). The performance of these mentally disadvantaged children is compared with the performance of 200 normally developing children by using a clinical procedure for estimating syntactic development in children's spontaneous speech. The analytical procedure followed is that presented in Laura Lee's "Developmental Sentence Analysis" (1974). At the present time the knowledge concerning dysfunctions of auditory language is based primarily on data from adults who have acquired language and subsequently lost a portion of it. Most of the literature speculates that mentally disabled children undergo a "delayed" language development and have a linguistic system that corresponds to younger normal children. The findings discussed in this paper indicate that there are significant linguistic differences between the speech of mentally disabled children and the speech of younger normal children. The two groups are paired by means of a developmental sentence score which determines overall sentence complexity. A four-year longitudinal study of a subgroup of these mentally disabled children indicates that there is a developmental plateau beyond which these children did not progress. (Author)