Primary hyperparathyroidism (HPT) is associated with cardiovascular disease, but there are no systematic studies on the electrocardiographic findings in HPT. Preoperative electrocardiograms from 139 patients with primary HPT and 97 control patients were therefore investigated. Serum calcium levels were found to be significantly correlated to the QRS amplitude (r = .26, P < .002), ST-segment duration (r = -.55, P < .0001), QT interval (r = -.64, P < .0001), and T wave duration (r = .19, P < .009). These relations were still significant when the influence of age, sex, and blood pressure level were taken into account in the multiple regression analysis. Thus, the electrocardiograms from HPT patients, compared to normocalcemic control patients, showed a shorter duration of the ST-segment, representing the systolic interval, and an increased amplitude of the QRS complex, representing ventricular muscle mass. Furthermore, the duration of the T wave was significantly prolonged in HPT patients. The hemodynamic effects of these findings have to be further evaluated.