Williams syndrome (WMS), a rare neurogenetic disorder, has been inthe forefront of research in cognitive psychology for the last ten years.WMS is characterized by a distinctive cognitive prole: mild to moderate mentalretardation with relatively and surprisingly good linguistic abilities, whileperformance on spatial tasks is extremely poor. Concentrating on the linguisticabilities of children and adolescents with WMS, studies of vocabulary developmentand grammatical development in 15 Hungarian WMS children are presented: childrenwere tested on tasks testing vocabulary, regular and irregular morphological;measures of nonword repetition and digit span were also obtained. In contrastto previous observations, results on the vocabulary task do not show thatuncommon words activated as easily for a WMS child as common ones. Resultsin a picture-naming task support that conforming to the normal pattern, uncommonwords are harder to retrieve. Results on the production of accusative andplural forms conrmed for Hungarian as well that regardless of the frequencyof the item, inected forms of irregulars are harder to produce, and oftenoverregularized in WMS, revealing a dissociation between the rules of grammarvs. the mental lexicon. Performance on rare words in the vocabulary task,and overall performance on the morphology task is associated to the capacityof phonological short-term memory: subjects with higher span perform betteron both tasks. The specication of the close relation between the capacityof phonological short-term memory and their linguistic measures awaits furtherstudy.