In a continuous series of 439 parathyroidectomies for primary or secondary hyperparathyroidism, 36 cases were reoperations. The initial operation had been a prior parathyroid exploration (PTx) in 17 cases, a thyroidectomy (Tx) in 17 cases, both operations in two cases. The incidence of parathyroid glands discovered at exploration was lower in both types of reoperations (67 % after Tx, 72 % after PTx) than in patients who were explored for the first time (94 %). Anatomical variations and parathyroid pathologies were comparable in patients reoperated after Tx and in those explored for the first time. In patients reoperated after PTx, anatomical abnormalities (46 % of glands in ectopic locations, 12 % of supernumerary glands) and multiglandular pathologies (56% of hyperplasias) were much more frequently encountered. Long term results were satisfactory in patients explored for the first time (98 % cures) and in reoperations after Tx (94 % cures), but were less favourable in reoperations after PTx (78 % cures).