Trauma-induced superior oblique palsy usually results from contusion or avulsion of the trochlear nerve or from decompensation of a congenital trochlear nerve palsy. Severe craniocerebral trauma is often associated with the former mechanism, whereas more minor closed-head injuries can decompensate a congenital phoria. We report a patient who developed an isolated trochlear nerve palsy following minor head trauma. Investigation revealed an unsuspected tentorial vascular malformation that was compressing the trochlear nerve in its subarachnoid course. In the absence of other features (e.g., documentation of old head tilt, large vertical fusion amplitudes) that support decompensation of a congenital phoria, compressive lesions should be sought in cases of fourth cranial nerve palsies that follow minor head trauma.