• We examined the usefulness of the Thought Disorder Index (TDI), an instrument for assessing 21 categories of thought disorder ranging from subtle to flagrant, in a group of schizophrenic patients treated with an antipsychotic drug. The TDI charted the changes in thought disorder and showed a high degree of congruence with other scales of thought disorder. Because it does not depend on the clinical interview for its data and because it yields sensitive information about thought slippage independently of symptom assessments, the TDI may be profitably employed in a variety of research contexts.