The aim of this study was to clarify the clinical, laboratory, and imaging findings of ovarian cancer in association with endometriotic cysts by detailed comparison of the findings of benign and malignant tumors. This was a retrospective study of 138 women who had an operation for ovarian tumors at the Department of Obstetrics and Gynecology of Kochi Health Sciences Center between September 1, 2011, and July 30, 2015. The ovarian tumors were divided into two groups: the benign group (endometriotic cysts) and the malignant group (ovarian cancer in association with endometriotic cysts). Of the 138 patients, 28 had malignant disease, and 110 had benign endometriotic cysts. Patients in the malignant group were significantly older than patients in the benign group. The mean maximum tumor diameter was also significantly larger for the malignant tumors. Unilocular-solid and multilocular-solid type tumors were present in 25.0% and 75.0% of malignant tumors, and in 9.1% and 19.1% of benign tumors, respectively. The mean maximum solid component diameter and height were significantly larger in the malignant tumors than in the benign tumors. The solid components were present on the abdominal side of the cyst wall in 12.5% of benign tumors and in 51.9% of malignant tumors. In elderly patients, the presence of large solid components in large endometriotic cysts, especially the abdominal side of the cyst wall, might suggest malignancy. The aim of this study was to clarify the findings of ovarian cancer in association with endometriotic cysts by detailed comparison of the findings of benign and malignant tumors. The presence of solid components in large endometriotic cysts, especially the abdominal side of the cyst wall, might suggest malignancy.