Primary hyperparathyroidism (HPT) is caused by a parathyroid adenoma, hyperplasia or carcinoma. Difficulties for the histologic diagnosis of abnormal parathyroid tissue are widely recognized. The aim of the study was to evaluate the reproducibility of the morphologic criteria through a concordance study among three pathologists. Representative slides of 40 patients with biochemically primary HPT stained with hematoxylin and eosin were blindly reviewed by three pathologists. Each pathologist established the diagnosis of adenoma or hyperplasia and assessed the presence of fat cells, a rim of normal tissue, a fibrous capsule, the number of cellular types, the lobular pattern, and the characteristics of the blood vessel's wall. A concordance analysis was then performed. Mean age of the group was 55 +/- 14 yr, 7 were males and 33 females. The concordance analysis among the three pathologists for the differential diagnosis between adenoma and hyperplasia, showed a Kappa index of 0.5. Kappa index for the presence of fat cells was 0.56, for the presence of a rim of normal tissue 0.47, and for the number of cellular types 0.29. The concordance for the differential diagnosis between parathyroid adenoma and hyperplasia in this study was low.