The presence of a cognitive deficit in patients with schizophrenia has been postulated for over a century, but has only relatively recently become generally accepted. Knowledge of the nature of the deficit may well lead to insights into the pathology of schizophrenia. However, the investigation of cognitive deficits is complicated by the presence of confounding factors, the most important of which is the antipsychotic medication taken by patients with schizophrenia. On a more positive note, if antipsychotic drugs have an effect on the cognitive processes of schizophrenic patients, this in turn may allow a better understanding of the mechanism of action of these drugs.In this article, we review 30 patient studies that investigated the effect of antipsychotic drugs on tests of cognitive function, including a series focusing on clozapine. The practical difficulties in designing patient studies of this type are profound. In particular, the differences between studies in terms of sample size, time of testing and type of statistical analysis are marked. The presence of important confounding variables is unavoidable; the studies reviewed varied greatly in the type and extent of the efforts taken to control for these variables. Although there is poor consistency between the results of these studies, a trend suggesting that antipsychotics improve cognitive function in patients with schizophrenia is discernable.The relationship between antipsychotics and laterality is also reviewed. Consistency between results is again poor, and no conclusions can be drawn at this juncture, although the possibility of progress using neuroimaging techniques appears to be promising.The anticholinergic effects of antipsychotics and other medications are also important, since it is likely that anticholinergic compounds have a detrimental effect on cognition. This can only complicate the investigation of the effect of antipsychotics on cognition, which may be mediated by action at several neuroreceptors. The damaging effect of anticholinergic compounds on cognitive function may have direct clinical implications.