INTRODUCTION: Structural right atrial abnormalities have been described in patients with arrhythmogenic right ventricular cardiomyopathy (ARVC). However, little is known about electrocardiographic signs of atrial involvement in ARVC because no systematic studies have been conducted. METHODS: P-wave-triggered signal-averaged orthogonal electrocardiogram from 40 ARVC patients (46 ± 15 years, 16 females) was compared with recordings from age- and sex-matched healthy control subjects for assessment of P-wave duration and morphology. P-wave morphology was classified with regard to the P-wave polarity in leads X, Y, and Z. RESULTS: P-wave duration was longer in patients (135 ± 18 vs 124 ± 12 milliseconds; P = .003). Two typical P-wave morphologies were identified in the controls: positive in X and Y and negative (45%) or biphasic (55%) in Z. In patients with ARVC , typical P waves were seen in only 60%, whereas 15 patients (37%) had atypical P-wave positive in all 3 leads (P <.0001). The presence of atypical P waves in the ARVC group was not associated with the presence of either structural or functional right ventricular abnormality. CONCLUSIONS: Patients with ARVC commonly demonstrate deteriorated atrial activation expressed either as prolonged P-wave duration or abnormal P-wave morphology. The P-wave abnormalities were not secondary to right ventricular dilatation. These findings show that atrial involvement is common in ARVC and may represent yet another manifestation of the disease to be considered for inclusion in ARVC diagnostic workup.