The serum fragment of cytokeratin 19 was measured in 130 patients with gastrointestinal, breast and gynecologic cancer and 62 patients with benign disease using CYFRA21-1 immunoradiometric assay. Patients with advanced cancer showed the highest level of serum CYFRA21-1. When the cut-off level was set at 2 ng/ml, 7 of 61 patients with earlier stage cancer were positive whereas 45 of 69 patients with advanced cancer had serum CYFRA21-1 levels over 2 ng/ml. In benign gastrointestinal and gynecologic diseases, 4 of 66 patients were positive. Retrospective evaluation revealed a good correlation between the serum CYFRA21-1 concentration and clinical course. CYFRA21-1 may be a useful tumor marker for monitoring of gastrointestinal and gynecologic malignancies.