A 41 year-old-female with mixed connective tissue disease (MCTD) associated with dissecting aneurysm (DA) of the right posterior cerebral artery is reported. This case satisfied the diagnostic criteria for MCTD, and cerebral angiography revealed the double-lumen sign and pooling of the contrast medium. Thus it was possible to make a definite diagnosis of both MCTD and DA. Fragility of the arterial wall due to the primary disease, MCTD, appeared to have played a role in the occurrence of dissection and the subarachnoid hemorrhage.