We found that urinary fibronectin (UFN) in cancer patients was almost all fragmented and consisted mainly of the cell-binding domain. We developed a two-site immunoenzymometric assay for UFN, using two monoclonal antibodies that both recognize this domain of fibronectin. The amount of UFN was expressed as arbitrary units per milligram of creatinine. Some 2% of the 623 healthy subjects had UFN above the clinical cutoff point (200 arb. units/mg creatinine), as did 14% of the 271 patients with nonmalignant diseases. In contrast, concentrations of UFN exceeded the cutoff point in 59% of the 589 patients with cancer. In 37 patients with gastrointestinal cancer tested for UFN and for one or more of three established serum tumor markers, UFN was found in 25, significantly more often than the other markers. These results indicated that UFN is a marker that may be helpful in evaluating many kinds of cancer.