It has been suggested that hemisphere-damaged patients with calculation disorders can be subdivided into 3 groups: agraphia or alexia for numbers, spatial dyscalculia and anarithmetia. The purpose of the present study was to evaluate the role played by visuospatial disorders and by anarithmetia in subjects with impaired calculation abilities. Seventy-six patients with focal hemispheric lesions and 26 normal controls with demonstrated ability to read and write numbers were given a written calculation task, the Token Test, the Crosses Test, a test of Constructional Apraxia and Raven's Progressive Matrices. In the calculation task, a quantitative score represented the number of digits that were correct numerically and put in correct position. A qualitative score with emphasis on visuospatial factors was obtained by scoring each problem with the criteria used in Benton's Visual Retention Test. Analysis of the results showed that both left and right hemisphere-damaged patients performed significantly worse than controls and that patients with left posterior lesions were particularly impaired even after correction of the acalculia scores by the results of the other neuropsychological tests. These results suggest that even though different factors may contribute to calculation disorders (impairment of intelligence, visuoconstructive difficulties and above all aphasia), left posterior lesions are particularly prone to produce an impairment in calculating abilities which is partially independent from the above disorders.