Thirty-two hospitalized patients diagnosed as schizophrenic were assigned in equal numbers to four groups on the basis of their scores on the Whitaker Index of Schizophrenic Thinking (WIST) and the paranoid (Pal scale of the MMPI. Subjects attempted to solve two conceptformation problems in which they were to identify relevant stimulus attributes, under two levels of demand on their memories for previous stimuli. Only in conjunction with a low score on the Pa scale did the schizophrenic index predict a conceptual deficit. Only the high-WIST, low-Pa group differed significantly from a control group of normal subjects. This result qualifies earlier research concerning the power of the WIST to predict logical performance. No difference was found between the two levels of memory demand.Conceptual behavior may be broadly defined as the collection of skills and knowledge that allows people to recognize regularities among objects and events, and to use those regularities to organize their cognitive representations of the world. Recent research (Bourne, Justesen , Abraham, Beeker, Brauchi, Whitaker, & Yaroush, 1977) has shown that the conceptual skills of a sample of patients diagnosed as schizophrenic varied as a function of the severity of schizophrenic thinking, as measured by the Whitaker Index of Schizophrenic Thinking (WIST) (Whitaker , 1973) .The WIST is a short (25-item) objective test that measures the illogicality of subjects' associations to single words, word pairs, and sentences that describe hypothetical inventions. Higher WIST indices indicate greater disturbance of thinking. In general, those patients who made unusual associations to the verbal stimuli on the WIST alsu failed to understand and abstract the relationship among visual stimuli required by a conceptual rule-learning task. Conspicuous,