BACKGROUND Clear cell sarcoma of tendons and aponeuroses is a rare mesenchymal tumor in childhood. It is difficult to treat and its prognosis is bad. METHODS A 9 year-old girl was admitted because of persistent pain in the left lower part of her thorax. Clinical examination showed a mass, 4 cm in diameter, embedded in the thoracic wall. Ultrasonography and CT-scan showed that this mass extended between the last ribs to the pleural cul-de-sac. There was no adenopathy and myelogram was normal. Biopsy of the mass showed features of clear cell sarcoma of tendons and aponeuroses. Cytogenetic studies showed translocation t(12;22) (q13-14;q12) in tumor cells. The patient was given chemotherapy followed by complete resection, but the tumor recurred locally 3 months later and the patient died, despite chemotherapy plus radiotherapy, 23 months after the apparent onset of disease. CONCLUSIONS Treatment of this type of tumor remains difficult. The existence of a break point on 22q12, as in Ewing sarcoma, suggests that this tumor in of neural crest cell origin.