This study explored two aspects of language expression (defining words and repeating sentences) in samples of 26 Downs Syndrome (DS) and 26 mentally handicapped (MH) adolescents and young adults matched on intelligence and chronological age. Testing included language tests, a comprehensive audiological assessment, and computer-based memory experiments. Multiple partial correlations (with the effects of intelligence quotient and chronological age statistically removed) suggested that, for both groups: (1) oral vocabulary and sentence imitation abilities were unrelated; (2) sentence imitation accuracy and speed were negatively correlated; (3) sentence imitation correlated with auditory short-term memory and sentence comprehension; and (4) neither oral vocabulary nor sentence imitation correlated with hearing sensitivity. DS subjects who were better at defining words and repeating sentences tended to perform better on an experimental task measuring speed of word identification. The results suggest that DS and MH expressive language differences emerge most clearly in contexts requiring precise remembering and repeating of word sequences, and that part of the DS difficulty may be rooted in the speed with which individual words are processed and recalled. (29 references) (JDD)