Background. Many patients with primary hyperparathyroidism (PHPT) present with less severe biochemical parameters. The purpose of this study was to compare the presentation, operative findings, and outcomes of these patients with ''mild'' PHPT to patients with ''overt'' disease. Methods. A retrospective review of a prospectively collected parathyroid database was performed to identify cases of PHPT undergoing an initial neck operation. Patients were classified as mild when either the preoperative calcium or PTH was within the normal limits. Comparisons were made with the Student's t test, Chisquared test, or Wilcoxon rank-sum test where appropriate. Kaplan-Meier estimates were plotted for disease-free survival and compared by the log-rank test. Results. Of the 1,429 patients who met inclusion criteria, 1,049 were classified as overt and 388 (27.1 %) were mild. Within the mild group, 122 (31.4 %) presented with normocalcemic PHPT and 266 (68.6 %) had a normal PTH. The two groups had similar demographics and renal function. Interestingly, the mild group had more than double the rate of kidney stones (3.1 vs. 1.3 %, p = 0.02). The mild group was less likely to localize on sestamibi scan (62.4 vs. 78.7 %, p\0.01). Intraoperatively, more mild patients exhibited multigland disease (34.3 vs. 14.1 %, p\0.01). When examining intraoperative PTH (IoPTH) kinetics where single adenomas were excised, the IoPTH fell at a rate of 6.9 pg/min in mild patients compared with 11.5 pg/min in the overt group (p\0.01). Accordingly, 62.2 % of patients in the overt group and 53.3 % in the mild group were cured at 5 min postexcision (p\0.01). There was no difference in the rates of persistence or recurrence between the groups, and disease-free survival estimates were identical (p = 0.27). Conclusions. Patients with mild PHPT were more likely to have multigland disease and a slower decline in IoPTH, but these patients can be successfully treated with surgery. © 2013 Society of Surgical Oncology.