Authors reviewed the literature on the ethology of saccular cerebral aneurysms. Nowadays the theories about inborn origin of the aneurysms are refuted. Numerous factors are involved in their formation. Two considered groups of the factors are the following: the hemodynamic ones and the ones related to particular cerebral artery histology. The first group comprises arterial hypertension and arterial blood flow disturbances in the brain. They increase the hemodynamic stress on arterial bifurcations. It has been also proven in the regions that the stagnation of blood flow may take place. It may induce a local arterial wall injury which may be caused either by anoxemic mechanism or by the release mediators from blood cells. The main factor in the second group is the particular make up cerebral artery walls, especially regions around their bifurcations, where a predominance of collagen fibers over elastic ones is found in connection with poorly developed muscular layer. The existing defects may be enhanced by disturbances in the process of connective tissue proteins synthesis which may be influenced by numerous endo- and exogenic factors. Apart from the genetic ones they may include: poisons like heavy metals and toxins present in cigarette smoke, medicines and hormones, especially steroidal ones. Unfortunately, experimental studies documenting the interdependencies between those substances and aneurysms are still scarce. Statistically proven risk factors of the malformations include: cigarette smoking, untreated arterial hypertension, female gender, collagen synthesis disturbances, and any disturbances in cerebral blood flow.