Both in Alzheimer's disease and in schizophrenia structural and functional changes in the frontal lobes are noted. They are manifested by an impairment of cognitive junctions associated with this cerebral region. The paper presents results of neuropsychological testing sensitive to the frontal lobe functioning in 30 patients with Alzheimer's disease and in 33 schizophrenics. The frontal lobe dysfunction was found to be more marked in patients with Alzheimer's disease than in those with schizophrenia. The intergroup differences were most pronounced in the Trail Making Test (Part B). Obtained neuropsychological data suggest also differentiation of the frontal lobe dysfunction in the two conditions.