Because of the wide spread of magnetic resonance image (MRI), it may be increasing to find a dissecting aneurysm (DA) of the posterior inferior cerebellar artery (PICA) that causes headache without associated hemorrhage or infarction. Generally, surgical treatment might be considered in cases of DA with hemorrhage. However, the treatment of an unruptured DA with headache or infarction has not been well established. This is the first report regarding the pathology of an intact DA of the PICA that presents with headache only, and the pathological findings showed interesting figures. A 44-year-old man with an unruptured DA of the left PICA presented with sudden left occipital headache. MRI revealed no hemorrhage or infarction. Magnetic resonance angiography (MRA) showed growth of the DA 12 days after the onset of headache. Therefore, surgery was performed to prevent aneurysmal rupture. This DA aneurysm was trapped and removed after an occipital artery (OA)-PICA anastomosis was performed. The surgery was performed without complication. Pathological findings showed folding of the internal elastic lamina, and the true lumen was torn by the false lumen. The dissection reached the adventitia and the wall had numerous macrophages. Pathological findings might help understanding the etiology of DAs and inflammation might play an important role in DAs.