23 patients with normal pressure hydrocephalus (NPH) underwent psychiatric examinations prior to and 80 days to 10 months after a ventriculo-peritoneal shunt operation. A global evaluation of the effect of the operation on the patients' mental symptoms indicated appreciable improvement in 10 cases and slight improvement in a further 4.8 patients were assessed as unchanged, and one as mentally deteriorated. The psychiatric analyses was based on a new diagnostic system created by two of the authors (G.L., H.M.). The patients manifested varying, often complex psychiatric symptom constellations with symptomatological components from two or more organic mental disorders. Before the operation a mild or a moderately severe somnolence-sopor-coma disorder (SSCD) was diagnosed in 10 cases. After the operation all these patients became free from symptoms of SSCD. This was the most unequivocal change in connection with the operation, and the elimination of all symptoms of SSCD was the single factor which most effectively contributed to the total therapeutical result in these patients. All 23 cases were considered to have symptoms of a more or less severe astheno-emotional disorder (AED) preoperatively. The degree of severity of this disorder could not be determined with satisfactory certainty in some of the patients with complex symptoms. Amongst the 17 cases where the preoperative symptomatology allowed for a reasonably precise calculation of the degree of severity of AED, 6 were assessed as markedly improved after the operation and 10 as largely unchanged. In one patient, symptoms of the AED increased when the postoperative course was complicated by a subdural haematoma. Symptoms of an emotional-motivational blunting disorder (EMD) were diagnosed in 5 cases before the operation. After the operation 3 of these patients were symptom free in this respect while 2 unchanged. Slight or moderately severe symptoms of Korsakoff's amnestic disorder (KAD) were before the operation found in 7 cases: at the postoperative examination 6 of these cases were improved, of which 4 were free from such symptoms; one was unchanged. According to our experience, confident prognoses concerning the effect of the shunt operation on symptoms of SSCD can be made preoperatively, while, for a particular patient, the therapeutic effect on AED, EMD and KAD is often difficult, or sometimes impossible, to foresee. This article contains three case reports which represent different forms and courses of the mental symptom patterns.