We report the study of a 23-year-old Italian girl, FF, with Down's syndrome (trisomy 21). FF showed a remarkably good developmental acquisition of Italian language and vocabulary and was able to learn English and French, although the latter with less proficiency. FF showed an entirely preserved function of the phonological short-term store and articulatory rehearsal components of verbal short-term memory. By contrast, she was impaired in a wide range of tasks assessing verbal and non-verbal reasoning, visuo-spatial perception and memory, and verbal long-term memory. These findings, in line with evidence from brain-damaged patients, normal subjects and children, suggest that phonological short-term memory plays a crucial role in vocabulary acquisition, which may occur in the presence of substantial deficits of general intelligence and episodic memory.