The reliability of arrhythmia detection in ambulatory ECG recordings with single vs. multiple leads has been examined. About one third of all records contain some beats in which one lead shows a dominant shape and another shows a nondominant shape. If only the first lead is analyzed, a false negative will result. Validation of the fully automated multilead TELAVIV system on the physician-annotated 48-case 2-lead MIT-BIH database has shown median and maximum errors of less than 1/2 and 2% respectively, in 40 cases which valid comparisons could be made.