The present study addresses the scope of the writing difficulties observed in patients with Alzheimer's disease (AD) and mild cognitive impairment (MCI). Patients with AD, patients with MCI and healthy controls performed a written picture-naming task and a direct copy transcoding task in which phonology-to-orthography (P-O) consistency was manipulated. Written latencies, inter-letter durations, mean word pen pressure and number of errors were measured. Results revealed that there was a significant difference between the control group and both groups of patients in written latencies and number of errors. However, in inter-letter interval or mean pen pressure there were significant differences only between the group with AD and the other groups. P-O consistency and the type of task produced similar effects in the three groups of participants, suggesting a general deficit affecting the ability to access/generate orthographic representations. The relevance of these results for understanding the spelling impairment in AD and MCI are discussed.