Oral and written picture descriptions were compared in 22 patients with Alzheimer's disease (AD) and 24 healthy elderly subjects. AD patients had a significant reduction of all word categories, which, similarly to controls, was more pronounced in written than in oral texts. They also reported fewer information units than controls, but without task difference. At the syntactic level, written descriptions of AD subjects were characterized by a diminution of subordinate clauses and a reduction of functors. More grammatical errors were present in written descriptions by AD and control subjects. AD and control groups produced an equivalent number of semantic errors in both tasks. However, in oral description, AD patients had more word-finding difficulties. In sum, AD descriptions were always shorter and less informative than control texts. Additionally, written descriptions of AD patients appeared shorter and more syntactically simplified than, but as informative as oral descriptions. Whereas no phonemic paraphasias were observed in either group, AD patients produced many more graphemic paragraphias than controls produced. Furthermore, written descriptions had more irrelevant semantic intrusions. Thus, as compared to oral descriptions, written texts appeared to be a more reliable test of semantic and linguistics difficulties in AD.