The method for diagnosing lactose intolerance in patients by the oral administration of defined amounts of lactose, followed by assaying metabolites of lactose, is performed in a way where the lactose administered contains 99% ¹³C-labeled lactose, preferably from 5 to 30 mg, and after the administration of the lactose, the content of ¹³C-labeled lactose is determined in a serum sample taken at a defined time after the administration. A diagnostic kit for performing the method consists of a capsule with from 5 to 30 mg of 99% ¹³C-labeled lactose, a patient instruction manual, a blood sampling device, sample containers for collecting the blood sample, and optionally a spatula and sample container for a stool specimen.