Tissue sections from 17 cases of parathyroid adenoma and ten cases of parathyroid hyperplasia were examined for mitotic activity. Mitoses were identified in 12 of the 17 cases of parathyroid adenoma and in eight of the ten cases of parathyroid hyperplasia. Clinical follow-up of all patients, even the four patients in whom numerous mitoses were present, showed no evidence of recurrent or aggressive disease. Mitotic activity alone is an unreliable indicator of aggressive potential in parathyroid disease.