The purpose of this report is to describe the seven-year clinical and neuroradiological follow-up of a perianeurysmal fatty lesion and perianeurysmal parenchymal cystic changes accompanying an asymptomatic unruptured wide-neck saccular aneurysm of the right middle cerebral artery trifurcation incidentally discovered in a 27-year-old male. The possible mechanisms of development are discussed. Notably, the close anatomical association between the two malformative lesions should rule out an incidental coexistence. Possible explanations include i) congenital structural deficiency of the cerebral vessel wall occurring at the same time of the formation of the fatty lesion, ii) fatty lesion inducing degeneration of the cerebral vessel wall possibly by limiting the nutrition of the smooth muscle cells, and iii) fatty lesion secreting some factors inducing weakness of the arterial wall to hemodynamic stress. Finally, both these lesion should lead to further impairment of perforating arteries resulting in areas of perianeurysmal encephalomalacia.