OBJECTIVES: Our study was designed to demonstrate that transesophageal echocardiography using a 3.3-mm monoplane probe can accurately evaluate the left atrium for patients with arrhythmias before cardioversion. BACKGROUND: Standard probes cause discomfort during intubation requiring sedation, but miniature probes do not. METHODS: With topical anesthesia alone, a 3.3-mm probe was used for transesophageal echocardiography in 60 patients. After intravenous sedation, standard transesophageal echocardiography was then performed. RESULTS: In 51 of 60 patients, the left atrium was visualized with the 3.3-mm probe. In 43 of 51 patients the appendage was clear. A thrombus was seen in 7 patients on both studies. In one patient spontaneous echocontrast was seen only with the 3.3-mm probe (sensitivity 100%, specificity 97%). In 9 of 60 patients, the appendage could not be assessed. CONCLUSIONS: In many patients the 3.3-mm probe can visualize the appendage and obviate the need for sedation. Technical advances will improve image quality with miniature probes.