Acute intracranial subdural hematomas (SDHs) of tentorial type generally pose no serious clinical threats, and unlike other variants of SDH, rarely require surgical intervention. Herein, we present an exceedingly rare case of tentorial SDH, marked by gradual enlargement and eventually calling for surgical evacuation. A 55-year-old man presented to the emergency department after sustaining head trauma. Initially, he was alert, fully oriented, and neurologically stable. Although computed tomography (CT) of the brain revealed an acute SDH scantily distributed along right tentorium, brain swelling or midline shift was negligible. On the following day, he became confused, but pupil size and light reflex remained normal. A follow-up CT scan showed considerable enlargement of the acute SDH, with midline shift. In a matter of hours, he deteriorated to a stuporous state, as the SDH enlarged even more. We performed a craniotomy and completely evacuated the SDH on an emergency basis. As a result, the midline shift improved, and he again became alert, soon recovering without any new neurologic deficit. This illustrative case demonstrates that even a tentorial SDH may ultimately deteriorate, forcing surgical evacuation. We, therefore, feel that close observation is mandatory for such events, even if the initial volume is small.