Data literatures report numerous association between giant congenital nevus and development alteration; only two cases describe its coexistence with thyroid disorders. However, we report the association of papillary thyroid cancer and giant congenital nevus. Papillary thyroid cancer is the most common differentiated thyroid cancer and has high prevalence in young women. In this paper we report: the case of a 18 years-old woman, affected by giant congenital melanocytic nevus on her back, who came to our observation because of one month of fever and increased volume of latero-cervical lymph nodes. Negative serologic tests allowed us to exclude lymphoma and mononucleosis. Because of the high risk (6%) that giant congenital melanocytic nevi could transform into malignant melanoma, we performed an ultrasound examination (US) of the cervical lymph nodes. The examination extended to the thyroid gland enabled us to visualize the same parenchyma alteration in both thyroid gland and lymph nodes. At last, fine-needle percoutaneus aspiration on thyroid lesion confirmed the presence of papillary carcinoma. In our case, thank to the optimal visualization of the parenchyma structure, US was diriment allowing a diagnosis of primitive thyroid lesion with an involvement of all lymph nodes in the neck. This findings legitimate the role of US as an accurate, noninvasive, radiation free and low-cost imaging technique in detecting differential diagnosis in the cervical lymphadenopathy, as well in preoperative staging thyroid carcinoma.