Research Diagnostic Criteria-diagnosed schizophrenic persons with average intelligence and dual diagnosis mentally retarded schizophrenic persons were tested in a forced-choice letter discrimination task in order to examine the relationship between schizophrenia and retardation from the standpoint of information-processing theory. The subjects consisted of eight schizophrenic persons of average or better intelligence, eight mentally retarded schizophrenic persons, and a control group of eight Research Diagnostic Criteria-diagnosed minor depressive individuals who were matched with the nonretarded schizophrenic group for intelligence. The groups did not differ significantly on the minimum exposure duration needed to identify an unmasked target stimulus at criterion levels of accuracy. When masked stimuli were employed, however, the depressive group obtained significantly more correct detections than both schizophrenic groups. More importantly, the performance of the two schizophrenic groups did not differ significantly. Our data indicated that schizophrenic deficits in information processing are independent of intellectual factors. Thus, vulnerability to a masking stimulus in schizophrenic persons can be attributed to the pathology of schizophrenia. This vulnerability indicates that schizophrenic patients are slow information processors.