BACKGROUND: Cellular fibronectin (cFN) is derived from the cell surface and extracellular matrix. It is fragmented by proteolytic enzymes, which is released from invasive and malignant tumor cells. It is circulated in blood and body fluids, and finally excreted in urine. This study was undertaken to determine the usefulness of plasma and urine cFN as a tumor marker of gastrointestinal tract cancers. METHODS: We measured the concentration of cFN in plasma and urine samples by Fibronectin EIA kit (Takara Shuzo Co., Ltd., Shiga, Japan). Subjects were 20 healthy adults, 20 patients with benign diseases, 51 patients with stomach cancer and 22 patients with colorectal cancer. We evaluated the clinical records to compare the concentrations of cFN with stage and degree of cancer metastasis. RESULTS: At the recommend cut-off values of 21 microgram/mL in plasma and 157 ng/mgCr in urine, the sensitivity of plasma and urine cFN was 41.2% and 43.1% in gastric cancers; 40.9% and 50.0% in colorectal cancers, respectively. The specificity of urine cFN was 92.5% as compared to 67.1% for plasma cFN. The cFNs in plasma and urine were significantly elevated in cancer group and the concentrations were increased further with peritoneal seeding and distant metastasis. The sensitivity of urine cFN was 73.3% in stomach cancers and 100% in colorectal cancers with distant metastasis. CONCLUSION: The urine cFN is specific and the concentrations increase further with metastasis, and may be useful in diagnosis and monitoring of gastrointestinal tract cancers. Further detailed studies must be made in a large number of patients.