We report on three patients with a documented small saccular aneurysm growing to a giant size over several years. In the first patient, a small dilation of the left carotid bifurcation grew to an asymptomatic giant aneurysm of 2 cm in diameter over 11 years. In the second patient, a ruptured aneurysm of the proximal middle cerebral artery (MCA) was erroneously treated by partial ligature of the MCA. Twenty-three years later, the aneurysm had grown to a giant polylobed calcified mass compressing the neighboring brain. In the third patient, a ruptured aneurysm of the dorsal part of the carotid syphon grew over 17 years to a giant calcified mass of 5 cm in diameter despite Hunterian ligation of the common carotid artery in the neck. The three reported cases illustrate the continuous growth and the long-term risks of not treating or of insufficiently treating aneurysms.