- Background Means for distinguishing the very common “benign early repolarization” from the very rare but malignant form are needed. Recently, the presence of early repolarization with “horizontal ST segment” was found to predict arrhythmic death during long-term follow-up in a large population study. We therefore speculated that the combination of “J waves with horizontal ST segment” would correlate with a history of idiopathic ventricular fibrillation (VF) better than the mere presence of J waves. Objectives To determine whether the morphology of the ST segment adds diagnostic value to the mere presence of J waves in a case–control series of idiopathic VF. Methods We reanalyzed our case–control study showing that the presence of J waves strongly correlates with a history of idiopathic VF among 45 patients with this disorder, 124 controls matched for age and gender (“matched-control” group), and 121 young athletes. This time we focused only on those patients with J waves and graded their ST-segment morphology as either “horizontal” or “ascending” according to predefined criteria. Results The presence of J waves was associated with a history of idiopathic VF with an odds ratio of 4.0 (95% confidence intervals = 2.0–7.9), but having both J waves and horizontal ST segment yielded an odds ratio of 13.8 (95% confidence intervals = 5.1–37.2) for having idiopathic VF. Conclusions We report, for the first time, that the combination of J waves with horizontal/descending ST segment improved our ability to distinguish patients with idiopathic VF from controls matched by gender and age.