We measured urinary levels of free L-fucose in healthy subjects, patients with benign diseases, and patients with cancer using an automated analyzer and a newly isolated L-fucose dehydrogenase, and evaluated the clinical usefulness of the results. The values obtained were corrected for urinary creatinine as micromoles per gram of creatinine. The cutoff value, set at the mean + 2SD for the healthy subjects, was 250 mumol/g.Cr. Patients with gallbladder cancer, bile-duct cancer, liver cancer, pancreatic cancer, or cirrhosis of the liver had significantly higher levels of L-fucose than the healthy subjects. The diagnostic sensitivity for these five diseases, taken together, was 68% (144/213). Specificity for the detection of cancer was calculated by use of false positives for patients with cholelithiasis, hepatitis, and pancreatitis: it was 73% (76/104). Diagnostic accuracy for these seven diseases taken together was therefore 69% (220/317). We compared the positive ratio of the L-fucose level with that of the tumor markers AFD and CA19-9. The positive ratio of an L-fucose value above the cutoff was higher than the positive ratio of either marker in bile-duct cancer, gallbladder cancer, liver cancer, and pancreatic cancer. The results suggested that the urinary levels of free L-fucose reflected the metabolism of sugar chains of glycoconjugates, and may be usefully clinically as a tumor marker.