An apparatus 1 for negative pressure wound therapy has a vacuum pump 10 and first 22 and second 26 chambers, the first chamber being in fluid communication with the pump. The negative pressure in the first chamber is prevented from exceeding a predetermined value of 120 mmHg by a precision orifice (30, figure 3) in the wall of a side arm 22C. The second chamber has a port 28 for a fluid connection with a wound. A one-way valve 24 between the first and second chambers prevents the second chamber from losing vacuum should pressure rise in the first chamber. A piezoresistive electronic pressure sensor 32 may be mounted in a side arm 26C of the second chamber and the vacuum pump deactivated when the sensed negative pressure is in the range 80-120 mmHg. Preferably, the vacuum pump is small (less than 10 cm3) and consumes less than 5W of power.
