Mevalonic aciduria is an inborn error of cholesterol and nonsterol isoprene biosynthesis due to mevalonate kinase de–ciency (MKD; McKusick 251170). Urinary excretion of mevalonate is massively increased. Clinical manifestations include psychomotor retardation, hypotonia, dysmorphic features, failure to thrive, cataracts and hepatosplenomegaly (Ho† mann et al 1993). In addition to the multisystemic symptoms, patients present recurrent febrile attacks accompanied by adenopathy, arthralgias, rash and diarrhoea. These recurrent episodes of unexplained high fever with the associated features are also a characteristic–nding of hyperimmunoglobulinaemia D and periodic fever syndrome (HIDS), a relatively benign condition in which elevated immunoglobulin D (IgD) itself appears not to be causative (Drenth et al 1994). The febrile episodes in HIDS have an average duration of 24h to 7 days, varying frequency, and a tendency to decrease in both numbers and severity of the attacks with age. We report 12 HIDS patients in whom a minimally increased mevalonate excretion during febrile episodes appeared to be related to mevalonate kinase (MK) de–ciency resulting from mutations in the gene encoding mevalonate kinase.