OBJECTIVES: Primary hyperparathyroidism (PHPT) is the third most common endocrine disorder, with the majority changed being asymptomatic. Parathyroidectomy (PTX), as the only definite treatment, is available to severe PHPT (~20%). We evaluated the mortality and co-morbidities in patients with PHPT with aim to estimate long-term impact of those untreated patients on the health care system. METHODS: This is a retrospective population-based cohort study. Using a unique patient identifier in Tayside, Scotland, electronic record-linkage enabled a dataset of PHPT patients to be created from an algorithm of biochemistry and nuclear scan records, hospital admission data and community based prescriptions. Three additional datasets were added in order to measure the mortality and co-morbidities in comparison with the general population using standardised mortality/morbidity ratios (SMRs) and standardised incidence ratios (SIRs). Patients were split into ‘mild’ and ‘aggressive’ groups broadly following the NIH criteria for PTX. RESULTS: During the decade of 1997 to 2006, we identified 2299 patients (70.4% female), 1683 (73.2%) as having mild PHPT.  A third of the aggressive PHPT patients (32.8%) had undergone PTX. We found an increased risk of all-cause mortality (SMR: 2.58, 95% CI (2.39-2.79)) as well as cardiovascular and cancer related deaths (SMR: 2.49 (2.21-2.80); 3.02 (2.60-3.48) respectively) in all patients.  The risk is similar between the mild and the unoperated aggressive groups, but slightly reduced in the operated group. Unoperated patients (both mild and aggressive) were also associated with high risk of co-morbidities, with renal disease being the highest. Surgery significantly improved patients’ health outcomes. CONCLUSIONS: The population prevalence of PHPT has generally increased; incidence is greater in females and increases with age. Mortality and morbidity are similarly higher for patients with mild and unoperated aggressive PHPT. Surgery may be worth recommending to all patients, and further study of the cost-effectiveness of surgery and chemotherapy is planned.