A 21-month-old girl suffering from tyrosinaemia type I and undergoing treatment with 2-(2-nitro-4-trifluoro-methylbenzoyl)-1,3-cyclohexanedione (NTBC) presented with pruritus which rapidly ceased with administration of high doses of ursodeoxycholic acid. Determination of plasma bile acids revealed clearly elevated levels both in samples taken before and after the onset of NTBC therapy, thus indicating, that the increase was not related to the administration of this drug. This result is corroborated by data from the first patient's newborn sister, diagnosed with the same disease, who showed elevated plasma bile acid concentrations in all samples examined, except for the cord plasma. This is the first report on altered bile acid concentrations in tyrosinaemia type I, and underlines the need for thorough investigation of bile acid metabolism in this disease.