Preliminary data are available on the coexistence of primary hyperparathyroidism and breast carcinoma. To further understand the association between hyperparathyroidism and breast cancer, we conducted a record‐linkage study in Sweden using the Swedish Cancer Registry from 1958–1997. A total of 9,835 women who underwent surgery for primary parathyroid adenoma were followed to evaluate the hypothesis that a history of primary hyperparathyroidism increases the risk of subsequent breast cancer. During 99,929 person‐years of follow‐up, 331 cases of newly diagnosed breast cancer were reported. The number of expected breast cancers in this population was 260.0. This resulted in a standardized incidence ratio of 1.27 (95% confidence interval [CI] = 1.14–1.41). The relation persisted over time after the surgical removal of the parathyroid adenoma. Possible explanations for the observed association are a shared etiology including genetic and environmental factors such as early life radiation, and hypercalcemia after the overproduction of parathyroid hormone, which may increase breast cancer incidence. © 2004 Wiley‐Liss, Inc.