Background: This study reports the language and memory progress over five years of 30 adolescents and young adults with Down syndrome, and investigates the relationship of earlier phonological memory abilities to later language development. Methods: Tests of nonverbal ability, receptive vocabulary, grammar comprehension, digit span and nonword repetition were administered at two points in time. Results: For the sample as a whole, there were significant gains in nonverbal ability, receptive vocabulary and grammar comprehension, but no increases in phonological memory measured by nonword repetition or digit span. However, there were considerable individual differences in progress which, in part, were related to chronological age. Phonological memory improved in many younger participants but there were signs of decline in some older ones. Partial correlations between earlier nonword repetition scores and later language scores, controlling for nonverbal ability and earlier language scores, indicated a significant role for phonological memory in the acquisition of vocabulary knowledge. There was similar evidence of a role for phonological memory in grammar comprehension, but only for younger participants. Earlier receptive vocabulary also predicted later nonword repetition scores, particularly for participants with higher levels of vocabulary knowledge. Conclusion: Relationships among the processes involved in language and memory development in Down syndrome may be similar to those established for typical development.