Brugada syndrome (BrS) and idiopathic ventricular fibrillation (IVF) have a higher incidence among males. Among genes coding ion channels, KCNE5 is located in the X chromosome, encodes an auxiliary subunit for K channels and modifies the transient outward current (I_to). In 205 Japanese patients with BrS or IVF, who were negative for SCN5A mutation, we conducted a genetic screen for KCNE5 and identified two novel KCNE5 variants, Y81H in 1 male and 2 female, and D92E-E93X in one male from 4 unrelated families. All probands received ICD implantation. Functional consequences of the KCNE5 variants were determined through biophysical assay using co-transfection with KCND3 or KCNQ1. In the experiments with KCND3, I_to was significantly increased for both of the KCNE5 variants compared to WT. In contrast, there were no significant changes in KCNQ1+KCNE5 WT and the two variants. With the simulation model, both variants demonstrated "notch and dome" or "loss of dome" patterns. In conclusion, novel KCNE5 variants increased I_to and appeared to cause ventricular fibrillation. Screening for KCNE5 is relevant for BrS or IVF.