BACKGROUND: Parathyroid carcinoma (PC) mimics benign primary hyperparathyroidism (PHP), but the diagnosis of PC is seldom available at the time of the first operation. Because PC is plagued by recurrences usually beyond cure, one may wonder whether some of these could be prevented by more extensive resections initially, i.e., if the diagnosis of PC were available at that time. METHODS: Over a 25-year period, 311 consecutive patients with PHP underwent operation in our department: 302 had benign disease (adenomas or hyperplasias), and 9 had PC. Several clinical parameters, serum calcium and parathyroid hormone (PTH) levels, and the weight of the parathyroid tumor removed were compared in both groups. Receiver operating characteristic curves and logistical regression analyses were used to distinguish PC from benign PHP. RESULTS: Eight of 9 patients with PC had symptoms, versus 238 (79%) of 302 with benign PHP (not significant). In the PC subgroup, serum calcium and PTH levels and the tumor weights of the parathyroid glands removed were significantly higher than in the benign PHP cohort, even if these three parameters were regularly flawed by low positive predictive values (14%, 20%, and 15%, respectively). CONCLUSIONS: Serum calcium, PTH levels, and tumor weights were significantly greater in the PC subgroup, even if not invariably in a discriminatory way. However, when PTH is <4 times the upper limit of normal and tumor weight is <1.9 g, the probability of PC is nil.