Background: The term fibromatosis or desmoid tumor refers to a group of benign fibrous growths without metastatic potential but with a significant risk of local recurrence. These lesions typically present infiltrative growth pattern with local invasion of adjacent tissues. This tendency is the reason for a relatively high rate of local recurrence, even after surgical removal. Fibromatosis is a very rare condition in general population but occurs more frequently in one of the familial cancer predispositions known as familial adenomatous polyposis (FAP) or Gardner syndrome. There are two main groups of fibromatosis: superficial (small, slow-growing lesions) and deep, also known as aggressive fibromatosis (large, rapid-growing lesions). Case Report: We report a case of a 6-year-old boy suffering from an aggressive form of fibromatosis. The patient developed a large pathological mass extending from the neck to the loins. After incisional biopsy and histpoathological examination of the sample, a diagnosis of aggressive fibromatosis was established. During the whole diagnostic process, different imaging techniques including CT, MRI and sonoelastography were used. As the surgical treatment was not possible, the patient was finally qualified for chemotherapy. Conclusions: Eventual diagnosis of aggressive fibromatosis is based on histopathological examination. However, it is an important condition that should be included in differential diagnosis of soft-tissue masses found in diagnostic imaging. Radiologists should be careful especially in defining the margins of infiltration in case of potential surgical treatment.