Association of hypertrophic osteoarthropathy and malignant thymoma is very rare in children. Authors report a case of a 14 year old girl followed for rheumatic fever since she was 8 years old. She was explored for arthralgia. The exam showed hippocratic fingers, soft tissues hypertrophy of lower limbs and dyspnoea. Plain radiography had demonstrated symmetrical ossifying periostisis in tubular bones of lower limbs evocating hypertrophic osteoarthropathy diagnosis. Chest radiography has demonstrated bilobate anterior mediastinal mass. Ultrasonography and MRI revealed a voluminous anterior mediastinal mass with cystic and fleshy components. Thymic adenocarcinoma diagnosis was made by surgical biopsy.