Primary hyperparathyroidism (PHPT) is thought to be a common disease in adults. However, it is a rare endocrine disorder in children and adolescents. We report two cases of primary hyperparathyroidism in children diagnosed at the Department of Endocrinology and Diabetes (EU and D) in the Children's Hospital (ChH), Kielce. The clinical course of the disease in these cases was fundamentally dissimilar, which confirms the observation that this rare endocrinopathy in children presents various clinical profiles, leading to diagnostic difficulties. In the first case, the severe course of PHPT was observed with signs suggesting a hypercalcemic crisis. In the second case, the patient was in a good condition with a mild hypercalcemia and symptoms limited to the skeleton, due to early identification of the disease. We believe these cases indicate the significant role of calcemia determination as a screening test in the diagnosis of PHPT, including in children.