Two sisters are shown who have been diagnosed as having glutaric acidity (GA) type I, in the neuroimage of which bilateral arachnoidal cysts were shown. There is a remarkable lack of correlation between the clinical status of the patients and the arachnoidal cysts. The younger sister developed the first symptoms of her illness at around 5 years of age, whereas the cysts had already been diagnosed at the age of one. It was considered convenient to doubt the diagnosis of type I GA, given the presence of bilateral or familiar arachnoidal cysts, despite the lack of symptoms.