The aim of our research was to find out whether cognitive-developmental tests such as Nominal Realism Test and Vygotsky Concept Formation Test could contribute to the process of diagnosing borderline and schizophrenic patients. The specific aim of this study was to assess the diagnostic power of subtests (such as Vocabulary, Comprehension, Similarities and Picture Arrangement Test) on Wechsler Adult Intelligence Scale (VITI) in the differential diagnosis of the two groups of patients. The study included 90 subjects, 30 of whom were diagnosed with borderline personality disorder (BPD), 30 had the diagnosis of schizophrenic psychosis (SCH), while 30, who had no psychiatric diagnosis, represented the control group. The findings indicate that the patients with BPD, and particularly those diagnosed with SCH, had both quantitative and qualitative cognitive impairment. The findings show that cognitive developmental tests represent valuable tools in the differential diagnosis of borderline personality disorder and schizophrenia.