An oral D-xylose tolerance test was carried out on 12 patients with portal liver cirrhosis, on 7 patients with active fatty liver disease and on 29 subjects without liver diseases. D-Xylose and D-threitol were measured by means of gas-liquid chromatography. Fifteen percent of the D-xylose dose excreted in urine within five hours was recovered as D-threitol. The proportion of D-threitol was greater when the collection was extended to 24 h. The D-threitol excretion was markedly diminished in cirrhotic patients, suggesting that a substantial proportion of the D-xylose-D-threitol conversion occurs in the liver. No decrease was detected in patients with fatty liver disease. No significant change in D-xylose excretion was observed in liver cirrhosis or in fatty liver disease. D-Threitol can be regarded as the main end product of D-xylose metabolism in man. The role of the glucuronate pathway in the D-xylose-D-threitol conversion is discussed.