Both mevalonic aciduria, characterized by psychomotor retardation, cerebellar ataxia, recurrent fever attacks, and death in early childhood, and hyper-immunoglobulin D (hyper-IgD) syndrome, with recurrent fever attacks without neurologic symptoms, are caused by a functional deficiency of mevalonate kinase. In a systematic review of known mevalonate kinase–deficient patients, the authors identified five adults with phenotypic overlap between these two syndromes, which argues for a continuous spectrum of disease. Mevalonate kinase deficiency should be considered in adult patients with fitting neurologic symptoms, with or without periodic fever attacks.