Tumor markers are useful tools in monitoring malignancies postoperatively or under hormone-/chemotherapy. In contrast, they usually lack diagnostic relevance and uncritical use may result in confusing situations. We describe three cases of diagnostic determinations of the tumor marker CA 125 resulting in subsequent partially invasive procedures. Based on these three cases, serum CA 125 levels were examined in 49 patients with abdominal diseases. We found CA 125 to be less a tumor product than an unspecific expression of stimulated mesothelial cells of the peritoneum. CA 125 was a marker for ascites (16 of 16 patients) and an indicator of infra-diaphragmatic involvement in non-Hodgkin's lymphoma (11 of 12 patients). Furthermore, 5 of 6 patients with inflammatory abdominal diseases showed elevated CA 125 levels, as did 13 of 15 patients with solid abdominal tumors of different histology (all non-ovarian cancer, no ascites). In conclusion, CA 125 remains a good marker for follow-up of ovarian cancer, but should not be used for diagnosis of abdominal processes.