A naturalistic observational single case study was carried out to investigate the form and function of private speech (PS) in a young man with Dandy-Walker variant syndrome and trisomy 22. Video recordings were observed, transcribed and coded to identify all combinations of type and form of PS. Through comparison between theories of PS and the results, five putative functions were identified in this case. In contrast to the predominant theoretical models of inner and PS, it is proposed that PS cannot necessarily be reduced to a single functional definition.