BACKGROUND A "natural language" measure was developed for classifying type and severity of communication disturbance in the speech of psychotic patients by assessing their linguistic reference performance. METHODS This measure was applied to speech samples of schizophrenic, manic, and nonpsychiatric subjects, and the groups were compared on levels and types of communication failures. RESULTS The speech of the schizophrenic and manic subjects contained much higher frequencies of each of six types of communication failures than did the speech of the control subjects. Proportions of the different types of unclarity differed among the diagnostic groups. CONCLUSIONS This method provides a measure of overall severity of communication disturbance, discriminates the speech of schizophrenic and manic subjects from that of nonpsychiatric subjects, and reflects some differences in distribution of types of communication failure in schizophrenic vs manic patients. The measure may be helpful in elucidating cognitive weaknesses underlying psychotic communication failures.