This report describes the case of a 2-year-old girl with desmoplastic fibroma of the mandible, with swelling in the region of the mandibular angle which had been wrongly diagnosed several times. A biopsy was interpreted as showing a low-grade fibrosarcoma. The patient then received treatment with cytotoxic drugs, and later a mandibular hemi-resection was performed. The postoperative diagnosis was Jaffé's desmoplastic fibroma (non-osteogenic). The age of the patient, the rapid development of the tumour and the accompanying pain suggested a sarcoma, and the first pathological examination seemed to provide confirmation. The post-operative course was very favorable, and the patient is considered to clinically cured, after one year.