The Octaray (Biosense Webster) is a novel, multispline mapping catheter with 48 closely spaced microelectrodes enabling high-resolution electroanatomical mapping. This study sought to report the initial clinical mapping experience with this novel catheter in a variety of cardiac arrhythmias and to compare the mapping performance with the 5-spline Pentaray. Fifty consecutive procedures among 46 patients were retrospectively analyzed regarding safety, efficacy, and acute procedural success defined as termination or noninducibility of clinical tachycardia, conduction block across an ablation line, or pulmonary vein isolation. In addition, another 10 patients with sustained atrial tachycardia mapped with the 5-spline catheter (2-5-2 spacing) or the novel 8-spline catheter (2-2-2-2-2 spacing) were analyzed. Left atrial and ventricular mapping by either transseptal (n=41) or retroaortic (n=2) access was feasible without any complications related to the multispline design of the novel catheter. The acute procedural success rate was 94%. In sustained atrial tachycardia compared with the 5-spline catheter, the novel 8-spline catheter recorded more electrograms per map (3,628 ± 714 vs 11,350 ± 1,203; P< 0.001) in a shorter mapping time (13 ± 2 vs 9 ± 1minutes; P=0.08) resulting in a higher point density (18 ± 4 vs 59 ± 10 electrograms/cm<sup>2</sup>; P< 0.01) and point acquisition rate (308 ± 69 vs 1,332 ± 208 electrograms/min.; P< 0.01). In this initial experience, mapping with the novel catheter was safe and efficient with a high electroanatomical resolution. In sustained atrial tachycardia the novel 8-spline catheter demonstrated a marked increase in point density and mapping speed compared with those of the 5-spline catheter. These initial results should be validated in a larger multicenter cohort with longer follow-up.