Four hundred forty-one patients who were operated on for primary hyperparathyroidism were observed for up to 22 years. Mortality was significantly higher than expected during the first postoperative years, for both men and women. This was predominantly due to a higher than expected incidence of diseases of the circulatory organs. The greater risk of death--compared with the expected mortality in the general patient population--appeared to decrease after the initial 5 to 8 postoperative years. However, for the entire follow-up period, the cumulative survival ratio (i.e., the ratio between the expected and the observed rates) was less than 1 for both sexes. The preoperative concentration of serum calcium did not influence the survival rates. The findings are consistent with the hypothesis that untreated hyperparathyroidism carries an increased risk of death, particularly from cardiovascular diseases but that this risk is gradually reduced after surgery.