Mevalonic aciduria represents the first documented inherited disorder of the pathway for the biosynthesis of cholesterol and non-sterol isoprenes in man (Hoffmann et al., 1986). Two patients have been described (Berger et al., 1985; Hoffmann et al., 1986), whose clinical presentations were very different. A deficiency of mevalonate kinase (ATP: mevalonate-5-phosphotransferase, E.C.2.7.1.36) activity was documented in extracts of cultured fibroblasts and lymphoblasts derived from the index patient and in lymphocytes isolated from whole venous blood (Hoffmann et al., 1986). The level of activity was <5% of controls. In contrast to patients with metabolic defects in catabolic pathways, the index patient with mevalonic aciduria due to mevalonate kinase deficiency did not suffer from recurrent acidosis triggered by excess of dietary nutrients or catabolic crises, but from a chronic multisystemic disease. He produced an enormous amount of mevalonic acid, which was excreted efficiently in the urine.