One hundred and forty-seven consecutive patients with primary hyperparathyroidism operted upon during the period of 1973 to 1978, have been studied with regard to incidence of neoplasms--malignant lesions, pituitary tumors and other benign tumors. The incidence was compared with that published in the Swedish Cancer Registry, which has been in existance since 1958. Patients who had undergone a cholecystectomy were used as another control group. The patients with primary hyperparathyroidism demonstrated 24 malignant tumors in a variety of organs, denoting an incidence of almost twice that calculated according to the Cancer Registry and that found in the control group. Sixteen instances of tumors of the pituitary were found--eight with hypersecretion of one or two pituitary hormones and eight with nonfunctioning adenomas. The incidence of neoplasms for the patients with primary hyperparathyroidism was 40, the expected incidence being 13. The neoplasms were predominant in the women. The data demonstrate that patients with primary hyperparathyroidism run an increased risk of having neoplasms develop. These may be encountered antecedant to, concomitant with or subsequent to the diagnosis of primary hyperparathyroidism.