Out of 334 patients operated on for PHPT [primary hyperparathyroidism] in the years 1956-1979, 34 (10.2%) had died before the end of the year 1980. In the sex and age matched control material of 334 patients operated on at the same time for varicose veins, appendicitis or hemorrhoids the mortality was 21 (6.3%). The difference in mortality between these groups was statistically significant (P < 0.05). The mean age at death of the PHPT patients was 65 yr and that of the control patients 67 yr. The PHPT patients who died differed from the whole PHPT material in higher mean age at operation (61 yr vs. 53 yr), higher preoperative serum Ca values (3.31 mmol/l vs. 3.08 mmol/l), more frequently elevated serum creatinine preoperatively (44 vs. 17%) and higher mean weight of the removed adenomas (3300 mg vs. 2000 mg). The PHPT patients who died had also more often the severe form of PHPT: 55% of the patients with hypercalcemic crises and 24% of the patients with cystic bone changes had died, whereas the respective percentage was 4% in the renal stone group. Four of the PHPT patients had died of uremic, and, in addition, 4 patients had progressive renal damage; 18 PHPT patients had died of cardiac disease, 4 patients of a cerebrovascular attack and 1 patient of other vascular disease. There were 8 cardiovascular deaths in the control group (P < 0.01). There were no differences in other causes of death, such as malignant tumors, between the groups. The results indicate that mortality was higher in the PHPT patients than in the controls and it has mainly been a consequence of the severe form of the PHPT disease at the time of operation. In addition, cardiovascular diseases as causes of death were excessively presented in the PHPT patients.