BACKGROUND Primary hyperparathyroidism during pregnancy is rare. The incidence of primary hyperparathyroidism in women of child-bearing age is estimated to be approximately 8 cases per 100,000 population per year. METHODS We provide a case report of a 33-year old pregnant woman, in whom primary hyperparathyroidism was diagnosed because of neonatal hypocalcemia two days after delivery. There was a history of recurrent urinary tract infections with hydronephrosis on the left side. The woman had been treated on an outpatient basis by an urologist, nephrostomy had been performed. After delivery, nephrolithiasis was diagnosed by x-ray and treated endoscopically. Parathyroid adenoma was found and removed successfully. CONCLUSIONS Due to physiological changes of calcium metabolism primary hyperparathyroidism is often without symptoms during pregnancy. Operative elimination of adenoma as a frequent cause of primary hyperparathyroidism reduces maternal and fetal morbidity and mortality. CONCLUSIONS In pregnant women with hydronephrosis on the left side primary hyperparathyroidism should be considered. If undetected, hyperparathyroidism causes considerable morbidity, not only for the mother but also for the child.