Mevalonic aciduria, due to mevalonate kinase deficiency, is the first recognized defect in the biosynthesis of cholesterol and isoprenoids. Very few patients with this disorder have been reported. Three siblings born from consanguineous parents are reported. Several clinical signs were present in all 3 children, including failure-to-thrive, susceptibility to infections, hepatosplenomegaly, cataract, and psychomotor retardation. Dysmorphic features were more apparent in the two older siblings. Urinary organic acid analysis by gas chromatography/mass spectrometry invariably revealed a high urinary excretion rate of mevalonic acid. Mevalonate kinase activity assayed in fibroblasts was very low. Diagnosis of this very rare disease may be suspected simply on clinical evidence; it is confirmed by abnormal excretion of mevalonic acid.