Neurofibromatosis type I (NF1) is an autosomal dominant multisystem disorder. Patients with NF1 are at increased risk for developing both benign and malignant tumours. We report the case of a patient with histologically documented NF1, who underwent F18-FDG-PET/CT for staging purposes. The study revealed intense uptake at multiple masses located at the thighs (the largest presented SUV max of 6.8), popliteal regions, legs, left foot, left supraclavicular region, and at the thoracic wall between the 11th and 12th right ribs. The surgical biopsy of the largest popliteal lesion with higher uptake at F18-FDG-PET/CT documented the presence of a malignant schwannoma at histological examination. In conclusion, F18-FDG-PET/CT was probably able to help the discrimination between benign lesions related to known NF1 and the malignant transformed ones, and to assist clinical decision making.