Two siblings with atypical methylmalonic aciduria and progressive encephalopathy are reported. Initial symptoms were failure to thrive and growth retardation from the first year of life, progressing to severe mental retardation, microcephaly, dystonia, spasticity and cataracts. The amount of methylmalonic acid excreted in the urine was substantially lower than in classical methylmalonic acidemia and was not reduced by vitamin B12 therapy. The activity of methylmalonyl-CoA mutase and the overall assay of propionic acid metabolism in cultured fibroblasts were normal. The primary defect in this probably new autosomal recessive disorder associated with methylmalonic aciduria is currently not known.