In 30 patients subjected to craniotomy, subdural pressure was measured with a 22G/0.8 mm Venflon cannula connected to a pressure transducer system. The measurements were performed after removal of the bone flap and just before opening of the dura. The subdural pressure was correlated with the tactile estimation of dural tension and the tendency to brain herniation after opening the dura. The results indicate that generally there is a poor agreement between the tactile feeling of dural tension and subdural pressure. Thus, in some patients with a relatively high dural pressure the dural tension was evaluated as relaxed. At subdural pressure below 6 mmHg brain herniation never occurred. On the other hand, at tensions over 7 mmHg some brain herniation occurred in all patients, and at tension over 11 mmHg pronounced brain herniation occurred. The method of subdural pressure used in this study is simple, the duration of the measurement is less than 1 min. It is concluded that measurement of subdural pressure before opening of the dura gives important information.