Pathological laughing, one subgroup of psuedobulbar affect, is known as laughter inappropriate to the patient's external circumstances and unrelated to the patient's internal emotional state. The authors present the case of a 76-year-old woman with no significant medical history who experienced pathological laughing after subarachnoid hemorrhage (SAH) due to rupture of an aneurysm, which was successfully treated with craniotomy for aneurysm clipping. In the acute stage after the operation she suffered from severe vasospasm and resulting middle cerebral artery territory infarction and conscious disturbance. As she regained consciousness she was afflicted by pathological laughing 6 months after the onset of SAH. Her involuntary laughter was inappropriate to the situation and was incongruent with the emotional state, and she could not control by herself. Finally the diagnosis of pathological laughing was made and treatment with sertraline, a selective serotonin reuptake inhibitor (SSRI), effectively cured the symptoms. Her pathological laughing was estimated to be consequence of infarction in the right prefrontal cortex and/or corona radiata, resulting from vasospasm. To the authors' knowledge, this is the first report of pathological laughing after aneurysmal SAH. The authors offer insight into the pathophysiology of this rare phenomenon. Effectiveness of sertraline would widen the treatment modality against pathological laughing.