Protein-bound fucose content in sera from normal persons and patients with various malignant and nonmalignant diseases was measured and statistically analyzed. Normal serum gave a mean value of 6.84 .+-. 0.13 mg/100 ml, and rarely exceeded 9 mg/100 ml. Although no significant difference was found between sexes, there was a tendency of fucose content to decrease in older persons. More than 90% of cancer-bearing patients had higher levels than the critical value (9 mg/100 ml), while only 8.7% of patients with benign tumor showed positive results. These results were not limited to special organs but were common to all cases studied. The elevation of serum fucose content in malignant tumors was well correlated with the stage of progression, though the levels were less significant in early and locally restricted breast and thyroid cancers. Serial postoperative follow-up study showed that the level of serum fucose was a useful diagnostic parameter for judging the effectiveness of therapy and the prognosis of the patient. The fucose content in malignant tumor tissue and metastasized lymph nodes appeared to be more elevated than that in normal tissue. The practical use and limitation of the fucose value in various diseases, together with a possible source of serum fucose, was discussed.