Primary hyperparathyroidism (PHPT) in multiple endocrine neoplasia (MEN) type IIa is rare, occurring in 20% to 30% of the patients. The aim of this study was to evaluate clinical findings, surgical therapy, and outcome for 56 patients affected by PHPT among 249 MEN-IIa patients collected from 84 families assembled by the Groupe d'Etude des Tumeurs á Calcitonine (GETC, French Calcitonin Tumors Study Group). This retrospective study was based on cases registered by the GETC (20 participating centers) from 1969 to 1994. Characteristics of PHPT in 56 patients (31 women, 25 men) with MEN-IIa were reviewed. All but two underwent cervicotomy. The median age at diagnosis was 37.6 years. PHPT was found concomitantly with medullary thyroid carcinoma (MTC) or pheochromocytoma in 43 patients (77%). PHPT was asymptomatic in 68% of the patients. Serum calcium levels ranged from 2.20 to 3.70 mmol/L (median 2.82 mmol/L; normal 2. 10-2.60 mmol/L). The number of parathyroid glands removed at surgery was 0 (n = 2), 1 (n = 24), 2 (n = 5), > 2 (n = 12), 4 (n = 11). Pathology (initial surgery) consisted of 24 adenomas, 4 double adenomas, and 25 hyperplasia. Cure after initial surgery was obtained in 89%, including a 22% incidence of hypoparathyroidism. There were 6 cases (11%) with persistent PHPT. With a mean follow-up of 6.4 years, five patients (9%) had recurrent PHPT. The results indicate that MEN-IIa-related PHPT is generally associated with mild, often asymptomatic hypercalcemia. Despite recurrences encountered 5 to 15 years after the first cervicotomy, resection of only macroscopically enlarged glands generally appears sufficient. Subtotal or total parathyroidectomy with autotransplantation is associated with a high rate of hypoparathyroidism.