A filter module of an extracorporeal blood treatment machine with a filter casing in which a hollow fiber bundle is attached whose hollow fibers comprise a hollow fibre wall made of a semi-permeable membrane, and whereby in their longitudinal extension the hollow fibers comprise a number of changes in their diameter. An amplitude of 0.1 to 1 mm is achieved for the exterior diameter of the respective hollow fiber starting from the hollow fibers axis, whereby the periodicity provided in the axial direction of the hollow fiber is within a range of 1 to 10 cm and on the inside of the filter casing at least one radial constriction is configured or provided by which the fiber bundle is locally constricted.
