An ultrafiltration device is designed to be used for ambulatory SCUF or other types of continuous or intermittent ultrafiltration, typically while being worn or otherwise carried by the subject (S) that is being treated. The ambulatory ultrafiltration device comprises a blood filter (4) that has a blood side (6) for fluid communication with the vascular system of the subject (S), an ultrafiltrate side (7), and a semipermeable membrane (8) disposed between the blood side (6) and the ultrafiltrate side (7). The ambulatory ultrafiltration also comprises a buffer vessel (3) in fluid communication with the blood side (6) of the blood filter (4), and a blood pump (9). The blood pump (9) is controlled to alternate between a withdrawal phase and a return phase. In the withdrawal phase, blood is withdrawn on a blood path from the subject (S) via the blood filter (4) to the buffer vessel (3). In the return phase, blood is returned from the buffer vessel (3) to the subject (S) on the blood path. The blood filter (4) is arranged to remove ultrafiltrate from the blood during at least one of the withdrawal and return phases. The use of a single blood path for withdrawal of blood and return of treated blood makes the device suitable for ambulatory ultrafiltration.
