OBJECTIVE: To evaluate the possible association between primary hyperparathyroidism and malignant neoplasms. DESIGN: An historical cohort study. SETTING: The only Regional General Hospital of the Province of Udine, Italy (population = 500.000). PARTICIPANTS: All the 101 patients with surgically treated parathyroid adenomas. MAIN OUTCOME MEASURE: The incidence rate of malignant tumor was calculated for this cohort based on the number of incidence cases and the person-years at risk. Standardized morbidity rate ratios (SMR) were calculated to infer the cancer relative risk of the study cohort as compared with the general population. RESULTS: A total of 13 cases of malignant neoplasms were ascertained among cohort members. The overall number of observed cases of malignancy did not exceed the number of expected cases (SMR = 1.0). However, strong and statistically significant direct associations were found with bladder cancer (SMR = 5.1) and polycythemia vera (SMR = 62.5). CONCLUSIONS: Due to the magnitude of the associations between parathyroid and bladder cancer and polycythemia vera, it is unlikely that they might be explained completely by bias or chance. Rather, biologically plausible explanations were identified. Particularly, non-paraneoplastic hypercalcemia due to primary hyperparathyroidism may increase the risk of these malignancies.