In a series of examinations of more than 1000 psychiatricneurological patients we tried to state the effects of diabetes mellitus on the nervous system. We attempted to diagnose the diabetic metabolic error by means of using the oral glucose tolerance test (100 g glucose), the plasma insulin levels and many other laboratory-technical examinations (altogether about 23 000 individual tests). It was found that 21,6% of the patients had a pathological glucose tolerance, 29.6% of them had a pathological insulin response and 5,5% of them showed a known and manifest diabetes mellitus. The comparison between the number of patients with a metabolic disturbance and the frequency of sick persons to be expected epidemiologically shows that an influence exercised by the diabetic metabolism may be stated in neurological and psychic disturbances of cerebral sclerosis and polyneuropathy. The central syndromes are more frequent than the peripheric ones. Mainly low plasma insulin is present. As a rule, pathological results may be seen more often during pathological glucose tolerance in electroencephalography, pneumencephalography, CSF-examinations, personality and intelligence states. This suggests that a diffuse disturbance upon the nervous system comes from the diabetic metabolism, without any acute metabolic disturbance which we did not find among our patients. This disturbance is evident clinically as so-called diabetic polyneuropathy and also encephalopathy. In opposition to that very often pathological changes can be seen by technical research methods in other neurological and psychiatric diagnoses without influencing essentially the autonomous progress of the psychiatricneurological illness.