Congenital long QT syndrome (LQTS) and catecholaminergic polymorphic ventricular tachycardia (CPVT) are known to be involved in some sudden unexplained death (SUD) cases. We examined possible mutations of the genes responsible for LQTS and CPVT in 17 SUD cases. Three cases showed RYR2 mutations, which are responsible for CPVT. We also found a novel KCNQ1 mutation. The KCNQ1 mutation is most frequently found in patients with congenital LQTS. Since morphological abnormalities have not been reported in patients with LQTS and CPVT, postmortem molecular screening for LQTS and CPVT-causative genes may be necessary for diagnosis of a SUD case. It may also useful for preventing living family members with the disease-causing mutation from cardiac events.