This study examined the cognitive functioning of 10 treatment-resistant schizophrenic patients after a minimum of a 1-year trial on clozapine. Results indicated significant improvements on prorated Wechsler Adult Intelligence Scale-Revised (WAIS-R) Full Scale, Verbal, and Performance IQs and on the WAIS-R Similarities and Digit Symbol subtests. A trend was also found for improvement on the Wisconsin Card Sorting Test. It was concluded that clozapine treatment is associated with global cognitive improvements in patients with treatment-resistant schizophrenia. Clozapine treatment may also improve performance on the Wisconsin Card Sorting Test in a subset of patients. Improvement did not appear to be related to the reduction of any cognitive effects of the typical neuroleptics upon their discontinuation.