Wisconsin Card Sorting Performance was examined in patients with complex partial seizures of left (n = 35) or right (n = 42) temporal-lobe origin. Our findings suggest that poor performance, in the form of perseveration, occurs in the context of nonfrontal dysfunction and depends not only on the laterality of the epileptogenic disturbance but also on the age of onset of the damage. The ability to shift set is most impaired by left temporal-lobe dysfunction, but only when the damage occurs very early in life. Left temporal-lobe dysfunction after one year of age leaves sorting behavior relatively intact. Perseveration may also occur in the context of right temporal-lobe dysfunction, regardless of the age of onset of the damage, although the deficit tends to be less marked. Finally, the occurrence of the deficit does not depend upon the status of the cerebral speech pattern or general intellectual level.