External ventricular drainage is prone to mechanical complications and monitoring the system is not easy. It requires repeated checking of the drip or the level of the drained cerebrospinal fluid (CSF) in the bag. It was thus proposed that an efficient method by which to monitor the complete external ventricular drainage system would be by continuously weighing the drained CSF. Using weighing scales, a converter, and a recorder, the CSF weight was displayed as a function of time (i.e., flow, if the specific gravity of the CSF is 1, which is usually the case as an approximation). This system was used in 16 comatose patients (11 with subarachnoid hemorrhages and 5 with hydrocephaly during 204 recorded periods of 22.7 +/- 2.8 hours (mean +/- standard deviation). The mean flow was 0.17 +/- 0.05 ml/min. A cessation of CSF flow was seen graphically as a plateau. This occurred 18 times because of obstruction, which was relieved before clinical worsening by making minor adjustments in 15 patients and by reinsertion of a drain in 3 patients. Nociceptive stimuli like tracheal suctioning were also clearly visible on the graph (increased slope, i.e., increased CSF flow). This type of external ventricular drainage monitoring appears to be safe and reliable, giving indications before changes in medical condition. The observed flow, however, is not equal to its rate of secretion but rather to the difference between CSF secretion and reabsorption.