Objective: Case reports have described associations between calcium metabolism disturbances and primary hyperparathyroidism with preeclampsia, suggesting parathyroid involvement in preeclampsia etiology. This study examines whether parathyroid adenoma, the main cause of hyperparathyroidism, diagnosed and treated before pregnancy is associated with preeclampsia. Design: We conducted a register-based study to assess the association between parathyroid adenoma and subsequent preeclampsia. Setting: Births among Sweden’s general population were studied. Population: The study population included 52 women with a diagnosis of parathyroid adenoma and 519 without, all of whom had a subsequent singleton pregnancy between 1973 and 1997. Methods: We performed a conditional logistic regression investigating the association of parathyroid adenoma with subsequent preeclampsia in the first singleton pregnancy with adjustment for potential confounding factors. Main Outcome Measure: The main outcome was a diagnosis of preeclampsia that does not include women with prior chronic hypertension. To ensure that treatment of parathyroid adenoma was completed before pregnancy, those with a diagnosis of parathyroid adenoma made less than 2 yr before delivery (and the matched comparison women) were excluded. Results: Statistically, parathyroid adenoma prior to delivery is significantly (P < 0.001) associated with preeclampsia, producing an adjusted odds ratio of 6.89 (95% confidence interval, 2.30, 20.58). Conclusion: A history of parathyroid adenoma should be viewed as a risk for preeclampsia. A history of parathyroid adenoma is associated with an increased risk for pre-eclampsia in subsequent pregnancies.