Ambulatory electrocardiographic monitoring (AEM) was performed in 22 patients (range 13-62 years; mean age 38.2 +/- 12.7) with grades I, II and III of myotonic dystrophy in order to evaluate the occurrence of potentially dangerous cardiac arrhythmias and conduction disturbances. All patients had previously undergone echocardiographic examination to determine whether structure and function abnormalities were present. In 6 patients with normal resting electrocardiogram, AEM revealed: first degree A-V block (4 cases), class IVa Lown ventricular arrhythmias (3 cases) and episodes of atrial fibrillation (4 cases). In 2 of 3 cases with abnormal scalar electrocardiogram new abnormalities (first degree A-V block and further prolongation of P-R interval) were demonstrated by AEM. Only 1 patient had mild signs of left ventricular dysfunction at echo. Disorders of cardiac conduction and rhythm are characteristic of myotonic dystrophy and can predispose to severe cardiac events. In this respect AEM is shown to be an early and sensitive tool in identifying patients at risk.