Periodic electrocardiographic RR intervals are often used to separate atrial flutter (AFl) from atrial fibrillation (AF), but have not yet been validated. We hypothesized that irregularly irregular electrocardiographic RR intervals poorly identify AF from AFl, and that atrial wavefront regularity affects variability in atrioventricular conduction. We studied the electrocardiographic RR intervals in 66 patients referred for ablation of AFl (21 atypical and 21 typical) and AF (n=24) in relation to intracardiac activation. Receiver-operating characteristics showed that periodicity in 60% of RR intervals provided only 66% sensitivity and 86% specificity for typical AFl and failed to separate atypical AFl from AF. Fewer RR intervals were integer atrial/ventricular ratios in atypical than in typical AFl (50% vs 83%, p=0.005), reflecting greater SDs in atrial cycles (6.98+/-7.02 vs 3.10+/-1.62 ms, p=0.023), more variable AH intervals (56+/-28 vs 18+/-21 ms, p=0.002), longer AH intervals (185+/-65 vs 118+/-31 ms, p=0.01), and greater AH variability for any atrial cycle length (p=0.026). In conclusion, irregularly irregular RR intervals are not specific for AF, are common in AFl, and should not be used to conclusively separate AF from AFl in the absence of sawtooth flutter waves. Future studies should explore the relation among AFl circuit location, cycle variability, and atrioventricular nodal conduction.