A series of patients with primary hyperparathyroidism showed a high incidence of anemia. Red cell morphology and indices, iron, folate and B12 were generally normal and reticulocyte counts inappropriately low. Focal fibrosis of the marrow space was observed in marrow biopsies from two patients, but no patient showed overt signs of a myelophthisic process. Anemia tended to occur in those with more severe hyperparathyroidism (higher serum calcium level or radiographic osteitis fibrosa cystica). Since severe hyperparathyroidism frequently compromises renal function, it is not always clear whether the anemia should be attributed to renal failure or to the metabolic disease. In nine of the 24 anemic patients, however, renal function was normal. Hyperparathyroidism may cause a hyporegenerative anemia, the mechanisms for which have not been elucidated.