Calculation and number processing abilities in 17 patients suffering from a mild form of dementia of Alzheimer's type (DAT) were studied by means of a standardized multitask assessment battery, the EC301 (Deloche et al., 1994). Patients were selected from a larger sample by using a specific visuo-perceptive task to control the confounding effects of deficits in analysing digit serial order. Language and memory skills were evaluated by means of standardized testing procedures. The EC301 overall score showed impaired performance in 12 cases. Calculation and number processing scores were highly correlated with Mini-Mental State Examination and language performance. However, multiple single-case analyses indicated heterogeneous patterns of preserved/impaired abilities with respect to the three cognitive areas under investigation (calculation, memory, and language) and to the different components of the calculation and number processing system.