Background: An accurate diagnosis of patent foramen ovale (PFO) and atrial septal aneurysm (ASA) may be of decisional importance in the management of patients with ischemic stroke. Very few studies have been devoted to observer agreement in the diagnosis of these atrial septum abnormalities using contrast transesophageal echocardiography, which is considered as the method of choice for the diagnosis. The aim of this study was to assess interobserver and intraobserver variability in the diagnosis of PFO and ASA with contrast echocardiography. Methods: Three sonographers independently reviewed 100 contrast studies stored on videotape on 2 occasions each. The interobserver and intraobserver variability was assessed by calculating κ statistics. Results: The overall interobserver and intraobserver κ values for the assessment of degree of shunting through a PFO were 0.77 (first and second reading) and 0.82, respectively. The best κ statistics were obtained when no and small shunts (less than 10 microbubbles) were pooled and compared with larger shunts. For the diagnosis of ASA, the overall interobserver κ value was 0.45 for the first reading and 0.71 for the second reading, whereas the overall intraobserver κ value was 0.74. Conclusion: Interobserver and intraobserver agreements for the diagnosis of PFO and ASA by transesophageal echocardiography are not perfect and need to be improved, particularly for ASA. This variability has to be taken into account when deciding on a potential risky treatment to prevent recurrent strokes. (J Am Soc Echocardiogr 2002;15:441-6.)