A new technic is described for the detection of left-to-right shunts. A miniature Geiger-Müller tube is placed in the esophagus behind the left atrium and positioned by the electrocardiogram. In this position the G-M tube measures radioactivity from the left atrium. Curves are then recorded simultaneously from the precordium, lung, and left atrium. Methods of injection, monitoring, and recording are presented that provide greater diagnostic accuracy over those external detector technics currently in use.Of 23 patients studied, 11 presented left-to-right shunts. Eight of these 11 patients were submitted to surgery; all were studied preoperatively and seven were studied postoperatively. Seven of the 11 shunts were detected by both precordial and esophageal technics. Four of the 11 shunts were missed by conventional precordial technics but were correctly diagnosed preoperatively by the esophageal detector. In addition, one shunt, which remained open postoperatively, was demonstrated clearly by the esophageal technic but was not indicated by the precordial curves. In no instance was the precordial curve positive for left-to-right shunt when the esophageal curve was negative.Although the accuracy of any radioisotope detection technic with external counting may not approach that of direct cardiac catheterization, this method, because of its relative simplicity and accuracy, is useful in the demonstration of left-to-right shunts both preoperatively and postoperatively.