RESUMEN
We report the case of a 21-year-old woman with a giant (20 cm) benign mesothelial splenic cyst and a high CA 19-9 serum level (1240 U/ml). The patient underwent resection of the cyst and splenectomy. True non-parasitic splenic cysts are very rare. Only 10 cases of benign true splenic cysts, with a high CA 19-9 serum level, have ever been published in the medical literature written in English. These are reviewed in this paper. We believe that the inner cellular lining of the cyst wall produces CA 19-9, which causes the high CA 19-9 serum level. This is supported by the facts that (1) the inner cellular lining is strongly positive for immunohistochemistry with CA 19-9, and (2) the CA 19-9 serum level returned to normal after resection of the cyst.