OBJECTIVE: This study aims to identify the best biochemical risk factors alongside other factors for predicting adverse outcomes seen in untreated primary hyperparathyroidism (PHPT). DESIGN: Population based cohort study, 1997-2006 SETTING: Tayside, Scotland, UK PATIENTS: Patients with untreated diagnosed PHPT OUTCOME MEASURES AND METHODS: Outcomes considered were all-cause mortality, fatal and non-fatal cardiovascular disease (CVD). Models were derived using survival analysis. Potential biochemical predictors tested were baseline serum calcium, parathyroid hormone (PTH), creatinine and alkaline phosphatase (ALP) and other covariates considered were gender, age at diagnosis, deprivation, previous co-morbidities and bisphosphonates usage. RESULTS: From 1997 to 2006, 2097 patients (mean age, 68.4 years; 69.9% female) with untreated PHPT were identified with a total follow-up of 7338 person-years, in the population of Tayside, Scotland. The baseline calcium was 2.65 mM and PTH was 10.3 pM. PTH was the only statistically significant risk factor in all outcomes observed adjusting for other covariates. Serum creatinine and ALP predicted mortality outcomes in the short term (=3 years) but not long term. Calcium was associated with increased risk of all cause mortality in the short term but had no significant impact on other outcomes. CONCLUSION: Baseline PTH, rather than calcium, best predicts long-term outcomes in untreated PHPT. © 2013 Blackwell Publishing Ltd.