Twenty-two consecutive cases of chronic subdural hematoma were treated successfully by twist-drill craniostomy after Tabaddor et al. and closed system drainage. The age distribution of the patients was between 44 and 83 years (mean of 64). Three of the 22 cases were female. A case of bilateral hematoma and six cases of high density hematoma (mean CT number 63.5 H.U.) were included. The drainage tube was kept in place for 3 days. Hematoma of 137 ml in average was drained on the first day, 74 ml on the second, and 22 ml on the third. The composition of the hematoma fluid drained on the first day was similar to that of peripheral blood, while that on the third day was similar to that of hygroma. In all cases, remarkable clinical amelioration was encountered soon after drainage, and no complications occurred. CT scans showed a marked reduction of the midline shift immediately after the drainage. The subdural space disappeared within a month in 14 cases and within 5 months in all except two elderly cases. In 13 cases, the subdural pressures before and at the end of drainage were measured. Among these, seven cases recorded higher pressure measurements at the end of drainage than at the beginning. An infusion test was performed in 12 cases by a bolus saline injection (4-10 ml, 1-2 ml/sec) followed by calculation of the pressure-volume index (PVI) and compliance (Co) according to Marmarou et al. Lumbar subarachnoid pressure was recorded in 10 cases. PVI ranged from 7.71 to 37.20, and Co from 0.37 to 2.30. Two cases which had high Co values (more than 3 S.D.) showed poor brain re-expansion in follow-up CT scans. In four cases, the infusion test revealed constantly higher pressure curves in the subdural space than in the subarachnoid. This means that a difference in tightness across the tentorium exists in patients with chronic subdural hematoma and that this may contribute to some extent to the onset of clinical symptoms and signs. This method has certain advantages, i.e. less surgical insult, freedom from tension pneumocephalus, and high precision in investigations of intracranial conditions under chronic brain compression in man. Even in two cases of poor brain re-expansion, this method was safely and effectively performed.