Background: The atrial fibrillation cycle length (AFCL) is a critical parameter for the perpetuation and termination of AF. In the present study, we evaluated a new method to measure the AFCL based on transthoracic tissue Doppler imaging (TDI) of the right atrium (RA) and left atrium (LA). Methods: Twenty patients with AF (6 acute AF, 14 persistent or permanent AF) were studied. A quadripolar catheter was positioned at RA or LA to measure AFCL (AFCLEGM, gold standard). Transthoracic echocardiography (apical 4‐chamber view) was used to perform pulsed wave TDI at the free wall of RA or LA. AFCLTDI was defined as the time interval between two consecutive positive to negative crossings of the baseline of the atrial time velocity curves. AFCLEGM and AFCLTDI were measured at baseline and during a 10‐minute infusion of flecainide (1.5 mg/kg). Results: Measurement of AFCLTDI was feasible in all but one patient. At baseline, AFCLEGM was 170 ± 22 ms, AFCLTDI 172 ± 22 ms (difference 2 ± 5 ms). AFCLTDI correlated significantly with AFCLEGM (R = 0.91, P < 0.0001). Bland‐Altman analysis showed a bias of −2 ms with a 95% limit of agreement between −26 ms and +22 ms. During flecainide, the AFCLTDI method yielded an AFCL prolongation from 176 ± 23 ms at baseline to 279 ± 68 ms (P < 0.01) after 10 minutes of infusion (57 ± 26%). Conclusions: (1) Tissue Doppler imaging of the atria during transthoracic echocardiography can be used to reliably determine the AFCL during both acute and persistent or permanent AF. (2) Continuous measurement of AFCL with TDI can be used to monitor the effect of antiarrhythmic drugs on atrial rate during AF. (3) This novel method is attractive because of the ease of acquiring the data and its noninvasive character.