In a prospective study, plasma levels of soluble fibrin (SF) were assessed in 97 patients with colorectal cancer immediately before and 1, 2, 7, and 90 days after surgery, 18 patients undergoing surgery for benign colorectal disease serving as controls. Age distribution, routine blood analysis, duration of surgery, perioperative blood loss and anaesthesia was similar in the two groups. SF was quantitated using a commercial enzyme-linked immunosorbent assay. The preoperative plasma level of SF was normal in cancer patients as a whole. However, patients with disseminated colorectal cancer had higher levels of SF preoperatively compared to patients with localized colorectal cancer (p < 0.01) and controls (p < 0.005). On days 1, 2, and 7 days postoperatively, a rather pronounced increase in plasma SF was observed in cancer patients as well as in the controls. Three months after surgery, plasma SF had normalized in controls and in patients undergoing curative cancer treatment. Postoperative deep venous thrombosis (DVT) was detected in 23% of the cancer patients by means of phlebography. The preoperative values of SF in these patients were higher compared to patients not developing DVT (p < 0.05). Patients with colon cancer displayed higher SF in plasma than patients with rectal cancer (p < 0.05).