A device and method for increasing the safety of an extracorporeal blood treatment machine are based on monitoring transmembrane pressure. The transmembrane pressure measured by a measuring device during the treatment is compared with an upper limit transmembrane pressure L.sub.1 and a lower limit transmembrane pressure L.sub.2 which define a monitoring window. If the transmembrane pressure is outside this monitoring window, an alarm is triggered by an alarm device. The monitoring window is shifted as a function of the ultrafiltration rate. To determine the window limits, the transmembrane pressure is established after a change in the ultrafiltration rate is calculated. Thereafter, the window limits are determined on the basis of the expected transmembrane pressure.