Introduction. Primary hyperparathyroidism is rare pathology in children (2?5:100,000). In more than 85% of patients, a single adenoma is present, and its extirpation is usually the only treatment a patient requires. In approximately 15?80% of cases, ectopic mediastinal parathyroid tissue can be found inside the thymus. Case outline. Our patient was a 13-year-old boy, who presented with multiple bone fractures in the previous period of time, and fatigue. Parathyroid hormone levels preoperatively were extremely high (1320 pg/ml ? more than 19 times higher than normal). Serum calcium was also elevated (total 3.55 mmol/l; ionized 1.41 mmol/l). He was examined and diagnosed as primary hyperparathyroidism by a pediatric endocrinologist. Imaging procedures for the preoperative localization of parathyroid adenomas were done (99mTc sestamibi scintigraphy and magnetic resonance imaging suggested ectopic mediastinal parathyroid adenoma). The patient underwent video-assisted thoracoscopic surgery procedure. After exploration of the mediastinum and chest, no ectopic parathyroid tissue was found, so total thoracoscopic thymectomy was performed. Final pathological section confirmed parathyroid adenoma inside the thymus. Conclusion. We believe that if no parathyroid tissue is found during surgical exploration of mediastinum, in a child with preoperatively detected parathyroid adenoma in anterior mediastinum, recommendation is to think about possible intrathymic localization and consider removing the thymus. Greater sample size is necessary for higher reliability of this statement.