Alexander disease (AxD) is a rare neurodegenerative disorder related to mutations in the glial fibrillary acidic protein gene. We report the case of a child with disease onset at the age of 3 months and a novel mutation in the glial fibrillary acidic protein gene. Peculiar aspects were initially atypical clinical and magnetic resonance imaging (MRI) findings, which became typical during follow-up. The child was born after an uneventful pregnancy, presented initially only as a failure to thrive. The first MRI examination demonstrated obstructive hydrocephalus and cerebral white matter abnormalities (which were more prominent posteriorly). During follow-up, her clinical picture became typical of AxD with macrocephaly and neurodevelopmental delay. Sequential MRI examinations showed frontal white matter involvement, together with exuberant forniceal lesions and areas of contrast enhancement.