We sought to determine the effect of neuroleptic treatment on thought disorder in chronic schizophrenics, using the Thought Disorder Index (TDI). We were able to demonstrate that high levels of thought disorder are found in chronic schizophrenic and schizoaffective patients, while in both bipolar patients and normal controls thought pathology is minimal. Moreover, the TDI appears to be particularly well suited to the characterization of thought disorder in chronic schizophrenics. For total scores on the TDI, we were unable to demonstrate a significant reduction in thought pathology, although it was evident that thought disorder as reflected in the Conceptual Disorganization and Unusual Thought Content subscales of the Brief Psychiatric Rating Scale is significantly reduced by neuroleptics. In analyses of subscores of the TDI reflecting four levels of severity, we were able to show that more severe forms of thought pathology are normalized by neuroleptics, while less severe pathology is largely unaffected. Our results suggest that neuroleptic treatment reduces thought pathology associated with acute episodes, but that somewhat milder, though still disabling, residual thought pathology persists after clinical remission has been attained.