['Primary hyperparathyroidism has been associated with premature death in cardiovascular diseases, but reversibility and background mechanisms are uncertain. This study investigates autonomic nervous function in hyperparathyroidism because disturbances in this respect have been related to increased cardiovascular morbidity.', 'Twenty-one consecutive patients with hyperparathyroidism and 23 matched control subjects without interfering medication underwent electrocardiographic recordings and ambulatory blood pressure (ABP) determination during a 24-hour period. Patients were re-examined at normocalcemia 1 year after parathyroidectomy. Heart rate variability was analyzed in subjects with sinus rhythm and normal atrioventricular conduction; time domain measures were calculated from the RR-intervals, and frequency domain variables were obtained with an autoregressive algorithm, which expressed power distribution (high, low, or very low frequency).', 'Nocturnal increase in low frequency was blunted in cases versus control subjects (103+/-128 m(2) vs 430+/-171 m(2); P=.03). Very low frequency at nighttime correlated inversely with serum parathormone (r=-0.33; P<.05) and increased postoperatively (P=.03). ABP was higher both at daytime and nighttime (P<.05) in patients and remained unchanged after surgery. Systolic ABP correlated with the parathormone (r=0.47; P<.01).'] Does circadian cardiac autonomic nerve dysfunction in primary hyperparathyroidism improve after parathyroidectomy?