Mevalonate kinase deficiency has been documented in an 8-year-old child who presented with cerebellar ataxia, hypotonia and mevalonic aciduria. The activity of mevalonate kinase in extracts of cultured skin fibroblasts derived from the patient was approximately 2% of the mean value for controls. Family studies were carried out on the mother, the father and a sister, all of whom were clinically well. Mevalonate kinase activity in extracts of cultured skin fibroblasts and transformed lymphoblasts derived from the parents of the patient were 43%-52% of the mean control values. These data are consistent with an autosomal recessive mode of inheritance for mevalonate kinase deficiency.