In a cardiac assist device, a housing has an inlet and outlet connectable to a patient's cardiovascular system. A pump is located within the housing to force blood that has entered through the inlet out through the outlet. The pump is connectable between the patient's left atrium and the aorta so as to assist the function of the left ventricle. A motor unit is also disposed within the housing, hermetically sealed from the pump. The whole unit is wholly implantable in a patient's chest. Permanently sealable valves, disposed in the inlet and outlet, may be closed without any invasive procedure to isolate the device from the heart once cardiac function assist is no longer required. The device may remain implanted within the patient indefinitely after the valves have been closed, and there is no requirement for a surgical procedure to remove the device once it is de-activated.