CA-125 is abundantly secreted from ovarian endometriotic cysts, but is not specific to endometriosis. In order to develop a new, more specific diagnostic marker for endometriosis, the iron concentrations in various ovarian cysts were assayed. The ovarian cysts were punctured and the contents aspirated laparoscopically, laparotomically, or transvaginally. The iron concentration in the ovarian cystic fluid was assayed using a spectrophotometer after protein precipitations and chromogen treatment. The iron concentrations in ovarian cysts were 69.5 ± 10.4 µmol/l in serous cystadenomas, 73.5 ± 29.3 in mucinous cystadenomas, 65.4 ± 12.4 in dermoid cysts, and 92.5 ± 18.2 µmol/l in adenocarcinomas. On the other hand, high iron concentrations were demonstrated in endometriotic cysts (1,749.6 ± 41.5 µmol/l), a lutein cyst (1,393.8), hemorrhagic corpus luteum (1,957.5) and endometrioid adenocarcinomas (1,860.9 ± 157.9 µmol/l). Cytological smear tests of the contents as well as histological examination allowed differential diagnosis between endometriosis and endometrioid adenocarcinoma. In conclusion, assay of the ovarian cystic iron concentration is a useful diagnostic tool for the evaluation of ovarian endometriotic cysts.