14 apparently healthy Leonbergers (median age, 18 months; range, 12e42 months). Severity of VA varied from infrequent couplets/triplets to frequent complexity (couplets, triplets, non-sustained ventricular tachycardias, (VTs) characterized by polymorphology. During follow-up, two dogs with polymorphic VT died. Although breed specificity and high prevalence indicate a heritable disease, based on available pedigree data, the mode of inheritance could not be determined. Conclusions: Sudden cardiac death in young Leonbergers is associated with malignant VA characterized by complexity and polymorphic nature. Diagnosis is based on 24-h Holter monitoring. Pedigree analysis suggests that the arrhythmia is familial.