The phonological abilities of two groups of 4-9-year-old intellectually impaired Cantonese-speaking children are described. Children with Down's syndrome did not differ from matched non-Down's syndrome controls in terms of a lexical comprehension measure, the size of their phoneme repertoires, the range of sounds affected by articulatory imprecision, or the number of consonants, vowels or tones produced in error. However, the types of errors made by the Down's syndrome children were different from those made by the control subjects. Cantonese-speaking children with Down's syndrome, as compared with controls, made a greater number of inconsistent errors, were more likely to produce non-developmental errors and were better in imitation than in spontaneous production. Despite extensive differences between the phonological structures of Cantonese and English, children with Down's syndrome acquiring these languages show the same characteristic pattern of speech errors. One unexpected finding was that the control group of non-Down's syndrome children failed to present with delayed phonological development typically reported for their English-speaking counterparts. The argument made is that cross-linguistic studies of intellectually impaired children's language acquisition provide evidence concerning language-specific characteristics of impairment, as opposed to those characteristics that, remaining constant across languages, are an integral part of the disorder. The results reported here support the hypothesis that the speech disorder typically associated with Down's syndrome arises from impaired phonological planning, i.e. a cognitive linguistic deficit.