Increasing evidence suggests that Lewy body disease (LBD) is associated with clinically important cardiac complications, including sick sinus syndrome, atrial fibrillation and sudden cardiac death. The high prevalence of sick sinus syndrome and atrial fibrillation in LBD suggests the presence of disease-related atrial conduction disorders. To explore whether LBD is associated with atrial conduction disorders, electrocardiographic (ECG) P wave parameters were analyzed in a cohort of LBD patients (n = 74), using age-matched Alzheimer's disease (AD) patients (n = 25) as controls. P wave terminal force in V1 and P wave duration were found to be significantly greater in the LBD group than in the AD group. In addition, 43 (58%) individuals in the LBD exhibited pathological P wave terminal force (> 4000 µV*ms) vs 3 (12%) in the AD group, and 60 (81%) in the LBD group exhibited pathological P wave duration (≥ 120 ms), vs 13 (52%) in the AD group. The difference could not be explained by atrial fibrillation or atrial enlargement on echocardiogram. The clinical significance of pathological P wave parameters in LBD is unknown, but their presence suggests an atrial cardiomyopathy that could be due to cardiac alpha-synuclein deposition, autonomic dysfunction, or a combination thereof. Future research is warranted to elucidate whether this proposed disease-related atrial cardiomyopathy is related to cardiac alpha-synuclein deposition, whether it is causally related to cardiac complications in LBD, and whether pathological P wave parameters hold potential as a screening tool for cardiac complications in LBD.