During a 3 month period we measured serum methylmalonic acid concentrations monthly in 37 patients, all on chronic haemodialysis because of end-stage kidney disease. Concentrations of methylmalonic acid in serum were above the upper reference limit in 36 of the 37 subjects. All patients were in regular cobalamin therapy, with intramuscular injections every third month, and all had normal to very high values of serum cobalamin. We found no normalization of serum methylmalonic acid during the examination period after cobalamin injections, and we could not demonstrate any relationship between concentrations of methylmalonic acid and creatinine, cobalamin and creatinine or methylmalonic acid and cobalamin in serum of these subjects. We conclude that an elevated serum methylmalonic acid concentration is a general finding in uraemic patients, and so the assay cannot be used to establish the diagnosis of tissue cobalamin deficiency in these patients.