Cerebral blood flow (CBF) was measured by the intra-arterial 133Xenon method in seven patients, aged 55 to 76 years, with chronic subdural hematomas. Before operation, CBF was reduced to an average of 31 ml/100g/min, range 24-38 ml/100g/min. One to 3 weeks after operation, when all had improved, CBF averaged 38 ml/100g/min, range 34-43 ml/100g/min. The reduction of CBF was probably secondary to a reduced metabolic demand. Clinical improvement continued for months after operation.