Organic psychoses are caused by a variety of disorders. In general, they are due to diffuse dysfunction of the brain without any specific anatomical basis, but they may be modified by disorders of some distinct neuronal systems as a result of local accentuation of a dissuse morbid process or localized brain damage. Acute organic psychoses are usually caused by disorders of the blood-brain barrier (cerebral oedema), acute neuronal dysfunction of disorders of synaptic transmission. Acute lesions may be reversible or terminate in stationary defective states or progressive neuronal degeneration. In late stages, the anatomical sequelae of the basic process and of secondary lesions are hardly to be separated. The non-specificity and inconsistency of brain lesions is demonstrated in chronic alcoholic psychoses. In senile organic psychoses there are quantitative correlations between psychopathological, neurophysiological (slowing of the basic rhythm) and morphological changes, mainly characterized by loss of neurons and synaptic contacts. Vascular syndromes are often over-diagnosed clinically. Localized mental syndromes are non-specific, but show characteristic locations, as they are commonly associated with dysfunction of neuronal systems engaged in storing and recall of information (frontobasal region and limbic system). Further detailed studies are needed in order to achieve better correlation between specific features of behavioural and intellectual defects and anatomical location and quantitiy of lesions.