This paper presents a detailed single-case study of a patient (BT) with dementia of the Alzheimer's type who showed impairments in number processing. Numeral comprehension and calculation abilities were largely preserved but the patient encountered substantial difficulties in transcoding tasks. In addition to syntactic errors, she produced numerous perseverations and intrusion errors, especially when she had to transcode arabic numerals into written verbal numerals and vice versa. In the present study, we show that these errors are concomitant with but not dependent on the syntactic deficit. We also demonstrate that their production is not exclusive to the numerical domain but clearly depend on the attentional processing load as well as on the familiarity of the transcoding task. To account for these data, we suggest that perseverations and intrusion errors are attentional in nature and originate from a unique impairment in selective attention capacities.