The present invention is directed to a specific mutation in SCN5A which causes drug-induced torsade de pointes or ventricular fibrillation. Persons with the mutation are predisposed to developing drug-induced torsade de pointes or ventricular fibrillation when administered certain drugs. This predisposition can be diagnosed in accordance with the present invention by analyzing the DNA sequence of the SCN5A of an individual. By screening patients for the mutation, drug-induced torsade de pointes or ventricular fibrillation can be avoided. Furthermore, drugs can be tested to determine whether they will cause torsade de pointes or ventricular fibrillation.