A method of discovering leakage in a heat exchanger, wherein the method is carried into effect on an intact heat exchanger which, together with surrounding conduits, is filled with water. The heat exchanger preferably has requisite pressure indicators mounted between each section, as well as requisite shut-off valves. The method comprises applying in each section of the heat exchanger an excess pressure on one side which is enclosed there, while the other side is at normal pressure. The pressure is enclosed during a period of time and is monitored during this period of time.