Dermatofibrosarcoma protuberans (DFSP) is a soft tissue tumor which may recur locally and rarely causes metastases to vital organs. DFSPs have specific chromosomal abnormalities involving the platelet-derived growth factor β-chain locus (PDGFB) which may render these tumors responsive to targeted therapy with the tyrosine kinase inhibitor imatinib mesylate. A patient with locally recurrent and metastatic DFSP resistant to first-line chemotherapy was treated with imatinib mesylate 400 mg/day. The tumor was examined by a novel fluorescence in situ hybridization (FISH) method for specific rearrangements of the PDGFB locus. The patient was followed for response and toxicity by physical examination and imaging studies. FISH revealed PDGFB rearrangement indicative of multiplication of the PDGFB fusion locus within a ring chromosome. Physical examination showed response within the first month of treatment, and subsequent computed tomography and fluorodeoxyglycose positron emission tomography documented complete response to imatinib therapy. Our patient is now in sustained complete remission for 20 months with minimal toxicity. We conclude that sustained complete remission of metastatic DFSP with specific FISH abnormalities involving the PDGFB locus can be obtained with imatinib mesylate with minimal toxicity for the patient.