Twenty-one patients with right hemisphere damage were studied (11 men, 10 women; average age = 41.33; range 19-65). Patients were divided into two groups: pre-Rolandic (six patients) and retro-Rolandic (15 patients) right hemisphere damage. A special calculation test was given. Different types of calculation errors were analyzed. Spatial alexia and agraphia for numbers, loss of calculation automatisms, reasoning errors, and general spatial defects interfering with normal number writing and reading, were observed. Number processing was impaired, whereas the calculation system was partially preserved. Fact retrieval and procedural difficulties were observed, but arithmetical rules were properly used. It is concluded that calculation abilities in right hemisphere damaged patients are disrupted as a result of: (1) visuospatial defects that interfere with the spatial arrangement of numbers and the mechanical aspects of mathematical operations: (2) inability to recall mathematical facts and appropriately to use them; and (3) inability to normally conceptualize quantities and process numbers.