OBJECTIVE: To evaluate the prognostic significance of isolated tumor cells detected by a panel of various monoclonal antibodies. SUMMARY BACKGROUND DATA: Previously, we showed by using immunocytology that cancer cells are frequently found in bone marrow and peritoneal cavity samples of gastrointestinal cancer patients. METHODS: Findings in bone marrow and peritoneal cavity samples were compared and correlated with the 4-year survival rate of 84 gastric and 109 colorectal patients with cancer. RESULTS: Although positive results in the bone marrow showed little prognostic significance, the peritoneal cavity results correlated with the 4-year survival rate (gastric cancer: p = 0.0038; colorectal cancer: p = 0.0079). Additionally, in subgroups of patients with early (gastric cancer: p = 0.02, colorectal cancer: p = 0.48) and advanced (gastric cancer: p = 0.02, colorectal cancer: p < 0.0001) tumor stages, a correlation of immunocytologic findings and the survival rate was seen. CONCLUSIONS: The detection of minimal residual disease in the peritoneal cavity serves as a new prognostic marker. Images Figure 5.