The ideal operation for primary hyperparathyroidism remains controversial. The postoperative serum calcium concentrations of two groups of patients who underwent different parathyroid operations at the University of Chicago Hospitals were studied. Group 1 consisted of 57 patients (operated upon prior to 1978) who had excision of an adenoma and any other questionably enlarged parathyroid glands and intentional biopsy of all other glands that could be identified. When hyperplasia was present, resection of three and one half glands was performed. Group 2 consisted of 50 patients (operated upon in 1980 to 1981) who had bilateral neck explorations and removal of the adenoma with biopsy of only one or two parathyroid glands. The last gland was not biopsied. When hyperplasia was present, excision of two glands with subtotal excision of two other glands was performed. Initial neck exploration cured 52 of 57 (92%) group 1 patients. In these individuals the postoperative serum calcium values fell to the same levels whether an adenoma or hyperplasia was present and whether one, two, or three glands were removed. During the first 4 postoperative days, 48% of these individuals had one or more serum calcium values of 7.9 mg/dl or lower. None had permanent hypoparathyroidism, however. Each of 50 group 2 patients was cured by the initial neck exploration. Serum calcium values of 7.9 mg/dl or lower occurred in 26% of these patients. Thus, transient postoperative hypocalcemia was decreased in both the adenoma and the hyperplasia groups by these more conservative operations. Recurrence has not occurred during this short follow-up period. While other studies have focused on glandular excision as the major cause of postoperative hypocalcemia, our data suggest that the excessive use of parathyroid biopsy may also have a deleterious effect.