The risk of thromboembolism after cardioversion of atrial flutter is controversial. The present study provides evidence for blood stasis in the atria of patients with atrial flutter and for gradual recovery of atrial contraction after cardioversion, which justifies prophylactic treatment at cardioversion, as for atrial fibrillation. We examined atrial thrombi and peak flow velocity in the left atrial appendage as an index of blood stasis in 5 consecutive patients with atrial flutter. Transesophageal echocardiography revealed a thrombus in 1 patient, and peak flow velocity in the left atrial appendage was inversely correlated with left atrial dimension (r = -0.90, p < 0.05). After restoration of sinus rhythm, transmitral flow velocity in late diastole was also examined to evaluate the recovery of atrial contraction. The recovery of transmitral flow velocity the next day and 1 week after cardioversion was correlated with flow velocity in the left atrial appendage before cardioversion (r = 0.89, p < 0.05; r = 0.97, p < 0.01, respectively). These findings suggest that some patients with atrial flutter have impaired atrial contraction and that prolonged impairment after cardioversion is also possible. Atrial enlargement and low flow velocity in the atrial appendage were predictive factors for such patients.