Background: Children and young people with Down's syndrome present with deficits in expressive speech and language, accompanied by strengths in vocabulary comprehension compared with non-verbal mental age. Intelligibility is particularly low, but whether speech is delayed or disordered is a controversial topic. Most studies suggest a delay, but no studies explore the relationship between cognitive or language skills and intelligibility. Aims: This study sought to determine whether severity of speech disorder correlates with language and cognitive level and to classify the types of errors, developmental or non-developmental, that occur in the speech of children and adolescents with Down's syndrome. Methods & Procedures: Fifteen children and adolescents with Down's syndrome (aged 9-18 years) were recruited. Participants completed a battery of standardized speech, language and cognitive assessments. The phonology assessment was subject to phonological and phonetic analyses. Results from each test were correlated to determine relationships. Outcome & Results: Individuals with Down's syndrome present with deficits in receptive and expressive language that are not wholly accounted for by their cognitive delay. Receptive vocabulary is a strength in comparison with expressive and receptive language skills, but it was unclear from the findings whether it is more advanced compared with non-verbal cognitive skills. The majority of speech errors were developmental in nature, but all of the children with Down's syndrome showed at least one atypical or non-developmental speech error. Conclusions & Implications: Children with Down's syndrome present with speech disorders characterized by atypical, and often unusual, errors alongside many developmental errors. A lack of correlation between speech and cognition or language measures suggests that the speech disorder in Down's syndrome is not simply due to cognitive delay. Better differential diagnosis of speech disorders in Down's syndrome is required, allowing interventions to target the specific disorder in each individual. (Contains 4 tables and 2 figures.)