Subdural hygroma is a frequent delayed complication of head trauma. It is a rare form of bleeding. Most hygromas are clinically silent and a few cases have shown slow deterioration in the chronic stage. We report a case of a 72-years-old male presented with a mild headache, consciousness disturbance and gait ataxia after head injury. Cranial computed tomography revealed subdural hygroma. The hygroma was associated to thrombocythemia. The patient was treated with chemotherapy with hydroxyurea with rapid resolution. In conclusion subdural hygroma may resolve spontaneously. Surgery might be deferred except in emergency conditions or in patients with normal neurological findings. Subdural hygroma in patient with thrombocythemia is first time presenting.