A battery of psychological experiments exploring various perceptual processes shows that the cognitive disturbances in Korsakoff''s psychosis is neither exclusively nor primarily in memorizing or learning. Patients suffering from this disease, even when capable of rational behavior and immune from confusional and confabulatory symptoms, are markedly defective in their perceptual functioning. Comparing them with controls drawn from psychiatric patients in alcoholic clinics and from hospitalized neurological patients not diagnosed as brain damaged or as alcoholic, the Korsakoff patients show a deficit on most perceptual tests. A notable exception to this observation are tasks which depend on the immediate apprehension of spatially and temporally unitary patterns or situations. The several discrete findings of this study can be subsumed most satisfactorily under a common principle by thinking of the Korsakoff patient as limited in his capacity to sustain more than a single perceptual set or attitude at a time, or to alter his orientation while the sensory input continues to confirm an expectancy he has adopted. This dependence on one aspect of the stimulus situation prevents him from integrating experiences, which may be the cause of his inability to learn or recall them; it also deprives him of those constant checking operations the absence of which has been proposed as an explanation of confabulatory behavior. The perceptual disorder appears to be a primary factor in the mental syndrome characteristic of Korsakoff''s psychosis; though indeed it may manifest jointly with other cognitive defects a common underlying attitudinal impairment.