The thymus cyst, which is difficult to recognize before the operation may occur in children as a painless swelling of the neck An asymptomatic course of about 100 cases has been reported in the English-written literature until 2002. The cyst is localized along the thymopharyngeal duct, i.e. from the mandible angle to the upper mediastinum. The authors describe cases of two patients. In the male patient the swelling resembled pathologically enlarged lymphatic nodes implicating malignant lymphoma. In the female patient the palpation examination of the swelling suggested the presence of lymphangioma. The radiological examination revealed a cystic formation or a liquefied lymphatic node. The operation on the patients revealed polycystic formation containing a clear brown liquid. Histological examination proved the formation to be a thymus cyst. The neck swelling in children may also be caused by developmental errors--a lateral cyst from the brachial arc, lymphangioma, hemangioma, medial cyst, thyroid gland cyst, parathyroid gland cyst, aberrant or ectoscopic thyroid gland, struma, laryngocoele, lymphatic nodopathy, pathologically changes salivary glands, phlegmona or abscess in the parapharyngeal space, teratoma on the neck, benign tumors (dermoid, epidermoid, neurofibroma, lipoma, lymphoma) and malignant tumors (sarcoma, lymph node metastases). The available literature does not describe any case of thymus cyst of the neck, which should be considered in differential diagnosis of swelling of the neck in children diagnosed before the operation. The final diagnosis may be established only after histological examination. Surgical extirpation is the primary therapy of the neck thymus cyst. The authors describe embryology, histology, present a survey of literature and deal with differential diagnosis, diagnosis and therapy of the neck thymus cyst in children.