We report a case of acute contralateral subdural hygroma (SDG) following decompressive craniectomy and discuss the potential aetiologies of the SDG. A 63-year-old man experienced drowsiness (Glasgow coma scale score 13) after a fall that resulted in head trauma. Brain CT revealed a subdural haematoma at the right fronto-temporo-parietal region with a midline shift to the left. Craniectomy for evacuation of the subdural haematoma was performed immediately. A delayed intracerebral haematoma with mass effect in the right frontotemporal region developed later, and was removed in a second operation. Although the patient's neurological status improved postoperatively, gradual deterioration was observed during the follow-up period. Contralateral SDG with a midline shift to the right was noted in a follow-up brain CT scan. The patient's condition improved after drainage of the SDG and he was discharged 1 week later.