Mevalonic aciduria is a rare disease that is a consequence of a deficiency of mevalonate kinase, an inborn error in the biosynthesis of cholesterol. Approximately 30 cases have been reported. We present our data on two siblings with mevalonic aciduria as a contribution to the recognition of this subject. Both were born after uneventful pregnancies. Their parents were healthy and not consanguineous. They had normal somatic and psychomotor development until they were around 2 years old. After the second year of life they developed mental retardation, ataxia and hypotonia. MRI showed cerebellar atrophy of both hemispheres and vermis. One sibling, from the age of 10 years onwards, suffered from complex partial seizures that were controlled with levetiracetam and lamotrigine. At 11 and 12 years of age, respectively, they were able to walk without help, but their gait was broad and ataxic. Their speech was dysarthric, fine motor skills were impaired as result of cerebellar ataxia, and they had moderate mental retardation. Diagnosis of mevalonic aciduria was made at this age through urinary organic acid analysis by gas chromatography-mass spectroscopy, which revealed high urinary excretion of mevalonic acid. They are currently 18 and 17 years old, respectively, show mental retardation and are able to walk but with difficulty. In our patients, ataxia due to cerebellar atrophy and mental retardation have been the predominant clinical manifestations. In mildly affected patients who survive infancy, these seem to be the predominant findings.