The aim of the present study was to determine whether patients with the incident hospital diagnosis of primary hyperparathyroidism (PHPT) in Denmark during the period 1977-1993 had an increased mortality from cardiovascular disease and cancer compared to the rest of the Danish population. In a random sample of half of the Danish population, all patients with an incident hospital diagnosis of PHPT were identified in the National Hospital Patients Register. The mortality in the cohort was analyzed and compared with the background population. A cohort of 1578 patients was identified. Follow-up of 1179 women and 376 men, who did not die in the same month as the incident PHPT diagnosis, was made through 1993. Of these, 312 (20%) died within the follow-up period. The standard mortality ratio (SMR) for women was 1.7 (95% confidence interval [95% CI]: 1.5-1.9). SMR for men was 1.6 (95% CI: 1.3-2.0). In women an increased mortality from ischemic heart disease, cerebrovascular disease, and cancer was found. In men an increased mortality from cerebrovascular disease and cancer was found.