1. The study aimed to investigate the effects of clozapine on the speed and accuracy of information processing in patients with schizophrenia. Data are reported from 13 subjects with schizophrenia, treated with clozapine for 6.8 (+/- 1.8) months. 2. Reaction time and accuracy of target detection on a tone detection task were measured before and during clozapine treatment, and these results were compared with a matched control group. 3. Symptom severity and performance on three timed tests of cognitive function were also measured prior to clozapine treatment in the schizophrenia group, and these measures were repeated during treatment with clozapine. 4. Treatment with clozapine was found to significantly improve reaction time and the accuracy of target detection in patients with schizophrenia. Despite this improvement their performance remained significantly inferior to that of a matched control group. Both positive and negative symptoms improved with clozapine treatment, as did performance on the WAIS-R digit symbol substitution test. 5. Improved performance on the WAIS-R digit symbol substitution test correlated with reduction in negative symptoms, and faster reaction time showed some correlation with reduction in positive symptoms. 6. The results of this pilot study indicate that treatment with clozapine can produce limited improvement in cognitive function in schizophrenia.