The indications for shunt operation in patients with idiopathic normal pressure hydrocephalus accompanied by brain atrophy (atypical idiopathic normal pressure hydrocephalus: AINPH) were investigated in 25 patients who satisfied the diagnostic criteria and underwent ventriculoperitoneal (VP) shunting. All patients had no apparent history of intra- or extracranial disease; dementia and gait disturbance as the main complaints; moderate to severe cerebral atrophy and ventricular dilatation and at least periventricular low density around the anterior horn on computed tomography; normal cerebrospinal fluid (CSF) pressure and filling of ventricles or cortical surface space with contrast medium at 24 hours on cisternography. The 15 male and 10 female patients were aged 47-83 years (mean 60.4 years). VP shunting was effective in 12 improved patients and not effective in 13 unimproved patients according to NPH grading. Pathological pressure wave on epidural pressure monitoring was observed in eight of 12 improved patients, but none of 13 unimproved patients. CSF outflow resistance was 35.33 +/- 11.16 mmHg/ml/min in improved patients and 9.12 +/- 3.51 mmHg/ml/min in unimproved patients. Preoperative serum alpha-1-antichymotrypsin value (alpha-1-ACT) was 42.02 +/- 8.64 mg/dl in improved patients and 61.72 +/- 11.03 mg/dl in unimproved patients. Alpha-1-ACT over 55 mg/dl occurred only in unimproved patients. Cerebral arteriovenous difference of oxygen content value (c-AVDO2) before and after surgery was 6.34 +/- 0.9 ml% and 5.91 +/- 0.78 ml% in improved patients and 4.75 +/- 1.85 ml% and 4.81 +/- 1.73 ml% in unimproved patients, respectively. The two cases with preoperative c-AVDO2 value over 8.5 ml% were both unimproved. Mean cerebral blood flow value before and after surgery was 23.51 +/- 4.20 ml/100 g/min and 45.22 +/- 8.11 ml/100 g/min in improved patients and 21.77 +/- 5.12 ml/100 g/min and 24.82 +/- 4.97 ml/100 g/min in unimproved patients, respectively. Cerebral atrophy in improved patients is caused by a cerebral circulation disturbance defined as a cerebral blood flow of penumbra or more due to cerebral arteriosclerosis, etc. A flow-chart of indications of shunt surgery for AINPH was prepared based on the results of the present study.