BACKGROUND/AIMS: The determination of serum carbohydrate antigen 19-9 (CA 19-9) level is useful in managing pancreatic cancer. However, the usefulness of this marker in evaluating the response to chemotherapy has not been fully established. MATERIALS AND METHODS: Serial changes of serum CA 19-9 levels were studied during chemotherapy in 66 pancreatic cancer patients who showed CA 19-9 level of 100 U/ml or greater before treatment. We investigated the relationship between patient survival and reduction in serum CA 19-9 level after treatment. RESULTS: When a responder was defined as a patient whose serum CA 19-9 level was reduced by more than 50% of the pre-treatment level within 2 months after treatment, CA 19-9 response was observed in 9 (13%) of the 66 patients. Median survival times of CA 19-9 responders and non-responders were 141 and 88 days, respectively. Based on Cox regression analysis, the relative risk of cancer death in CA 19-9 responders for non-responders was 0.47 (95% confidence interval, 0.21 to 1.05). CONCLUSIONS: CA 19-9 reduction may be useful for assessing the efficacy of chemotherapy for advanced pancreatic cancer.