The sizes of the right atrium (RA), cavotricuspid isthmus, and Eustachian valve are predictors of success of radiofrequency catheter ablation for atrial flutter (AFL). We examined the relationship between the sizes of cavotricuspid isthmus as measured by multidetector-row computed tomography (MDCT) and fluoroscopy. We used eight-detector MDCT to measure the tricuspid isthmus of 23 patients prior to linear ablation for common AFL. One patient with a deep pouch in the RA was excluded. Parameters measured were (1) the length of the trace of isthmus (Ti), which was equivalent to the blocking line; (2) the size of the tricuspid isthmus (DTi); and (3) the distance from the tricuspid valve and inferior vena cava (IVC) (LDTi). DTi and LDTi indicate the size of the RA, reflecting the appropriately sized steerable ablation catheter, respectively. Of the 22 patients, 21 were ablated successfully without recurrence of AFL, and clinical success was achieved in one additional patient despite failure to obtain a bidirectional block. Ti, DTi, and LDTi were correlated with fluoroscopy time (r = 0.84, r = 0.88, and r = 0.88, respectively; P < 0.0001), total delivered energy (r = 0.81, r = 0.80, and r = 0.83, respectively; P < 0.0001), and application time (r = 0.84, r = 0.80, and r = 0.87, respectively; P < 0.0001). Measurement of the tricuspid isthmus by MDCT may noninvasively provide important information for successful linear ablation.