A method for diagnosis of isolated tracheo-oesophageal fistula by oesophageal manometry is described. It consists of pull-through registration of intraluminal pressures. An abrupt pressure rise in the area of the fistula during inspiration is pathological. In principle, this method consists of the graphical tracing of the original Koops test which we have slightly modified. The investigation is non-invasive and accurate in comparison with roentgenological and/or endoscopical methods.