In this paper we describe the construction of a Graded Difficulty Arithmetic test (GDA) consisting of 12 additions and 12 subtractions which are orally presented. The test was administered to a control group of 100 volunteer subjects with extra-cerebral neurological disorders and to two experimental groups of patients with unilateral cerebral lesions of the left and right hemisphere. In the control group performance on the GDA was found to correlate highly with other measures of verbal intelligence, namely the National Adult Reading Test, the WAIS Arithmetic subtest and the WAIS Digit Span subtest. Between group analysis showed a significant groups effect on the GDA, the left hemisphere lesion group showing greater impairment compared to the right hemisphere lesion group and the controls. Using "cut-off" scores the left hemisphere lesion group's performance was shown to be significantly worse than that of the right hemisphere lesion group, who in turn were not significantly worse than the control group.