A total of 37 cases of ovarian primary squamous cell carcinoma (SCC)-19 associated with a dermoid cyst (SCCD), seven associated with endometriosis (SCCE), and 11 pure (SCCP)-are described. The last 18 cases belong within the new World Health Organization category of SCC in the surface epithelial-stromal category. The 19 patients with SCCD were 21-75 (mean, 52) years old; three of the carcinomas were in situ and seven, six, and three tumors were stages I, II, and III, respectively. The tumors and associated dermoid cysts were 6-35 cm in greatest dimension, usually forming mural nodules with intracavitary protrusion and focal necrosis and hemorrhage; two, seven, and seven tumors were grades 1, 2, and 3, respectively. SCCD was focally associated with a columnar epithelial cyst lining in 13 cases, suggesting an origin therein. One patient with stage I, grade 1 SCCD also had squamous cell carcinoma in situ (CIS) of the cervix. The seven patients with SCCE were 29-70 (mean, 49) years old, and one, three, one, and two tumors were stages I, II, III, and IV, respectively; all of the tumors were grade 3. One was associated with squamous cell carcinoma in situ of the cervix. The 11 patients with SCCP were 27-73 (mean, 56) years old, and one, four, five, and one tumors were stages I, II, III, and IV, respectively. The tumors were 6-26 cm in greatest diameter, usually solid with focal necrosis; one and 10 tumors were grades 2 and 3, respectively. Three patients with SCCP also had cervical squamous cell carcinoma in situ. The patients with SCCE had a poorer overall survival than those with SCCD. Five of the six patients with SCCE for whom adequate follow-up information was available died of their disease (mean survival, 5 months); also, in all five cases of SCCE reported in the literature, the patients died of their disease (mean survival, 4 months). The stage of the tumor and its grade correlated best with overall survival for all three types of SCC.