From 1960 to 1969 we have extended to a total number of 2670 diabetic patients our investigations on type and frequency of major psychosis associated with diabetes. Cases of Bleuler’s aspecific endocrine psychosyndrome and of neurosis or transient emotional disorders, as well as of presenile and senile dementia have not been consid­ered in this study, 12 cases of manic-depressive psychosis have been collected, after exclusion of involutional melancholic syndromes; in 2 subjects recurrent manic monopolar episodes, with striking social reflexes followed. Suicide was attempted in 1 case, 2 cases of paranoid schizophrenia have been found: one with ambitious and persecutory delirium, the other with persecutory delirium and physical delusions; another patient presented mixed psychosis of schizophrenic-melancholic type. In regard to the implications of psychosis in the clinical course of diabetes, we have pointed out two observations: a) one case which presented a depressive stuporous state and catatonia causing a false admission-diagnosis of diabetic coma; b) one case of paranoia showing a progressive worsening of the diabetes leading to a lethal hyperosmolar non-ketotic coma. Our results indicate the possible importance of paranoid schizophrenia among the various psychosis associated with diabetes and may afford some opportunity of discussion on the metabolic theories of schizophrenia. The remarkable incidence of cyclothymia in diabetes might give rise to the old hypothesis of common endogenous pathogenesis of the two diseases.