We herein report a gossypiboma resulting from a retained surgical swab, which had been left in peritoneum for 20years after appendectomy. CT revealed a cystic mass with a calcified reticulate rind. Subsequent surgery and pathological examination showed a gossypiboma. A simple experiment, using a barium-soaked surgical swab demonstrating similar CT appearance, supported our postulation that calcium deposition on the reticulated fibers of a surgical swab could generate such a characteristic calcified reticulate rind sign. We believe that identification of this CT sign facilitates the diagnosis of gossypibomas.