A case of adenoma-based hyperparathyroidism operated at the 4th month of pregnancy (third reported case) is described. The clinical picture was extremely flimsy and diagnosis was formulated exclusively on the observation of the characteristic alterations in the calcium-phosphorus metabolism. The validity of examining renal phosphate excretion for the diagnosis of hyperparathyroidism is considered and proposed evaluation indices (P.E.I. of Nordin and Fraser; I.P.E. OF Nordin and Bulusu) are discussed. The gravity of foetal prognosis in hyperparathyroid women is sufficient justification for surgery during pregnancy.