RESUMEN
INTRODUCTION: Marfan's syndrome is a hereditary, autosomal dominant multisystemic disorder involving connective tissue. Bilateral extensor carpi ulnaris and ulnar nerve (UN) instability is rare, usually caused by the alteration of structures mainly formed by connective tissue. The association between Marfan's syndrome and bilateral instability of UN and extensor carpi ulnaris has never been reported. CASE REPORT: We present the case of a 38-year-old female with no history of trauma, diagnosed with Marfan's syndrome, who developed bilateralinstability of the UN and extensor carpi ulnaris. Bilateral UN transposition and extensor carpi ulnaris tenoplasty were performed. CONCLUSION: Atraumatic bilateral instability of UN and extensor carpi ulnaris is a new rare clinical profile caused by Marfan's syndrome whether standard treatment is successful in a long-term basis in these particular cases of collagen intrinsic pathology remains unclear.