Endometriosis (EMs) is a benign disease characterized by the presence of endometrial tissue outside the uterine cavity. EMs associated with ovarian cancer (OC) has a relative low incidence (5% to 10%), sometimes with evidence of a transition stage through atypical EMs (1.6% cases). We have assessed 135 consecutive patients with either EMs or OC and, out of them, our study reports on four cases of ovarian EMs and OC: two cases with endometrioid OC and two cases with high-grade serous OC (HGSOC). Cases with EMs and HGSOC are extremely rarely reported in the literature - we could find not more than 30 cases. The main objective of our research was to observe the possible similarities between EMs and OC. Secondly, we analyzed the differences between EMs associated with endometrioid OC and EMs associated with HGSOC. We evaluated them in terms of clinical status (age, stages of EMs and OC) and immunohistochemical (IHC) expression of estrogen receptor (ER), progesterone receptor (PR), Ki67, p53, p16, Wilms' tumor 1 (WT1), cluster of differentiation (CD) 34 and CD10 immunomarkers - we could not find in the literature all these markers assessed, in the same time, to such samples. Our results indicated that there are no similarities between EMs and OC and no atypical EMs was identified in our cases. We recorded higher values of ER expression in EMs associated with HGSOC than in EMs associated with endometrioid OC. Higher values of ER expression were also recorded in OC than in endometriotic foci. There were no differences in proliferative rate of endometriotic foci associated with endometrioid OC, compared to EMs associated with HGSOC. An aberrant IHC expression for p53 protein and p16 protein was noted only in HGSOC. Also, a positive immunostaining for Wilms' tumor 1 (WT1) was identified only in HGSOC. Higher values of microvessel density were recorded in OC but not in endometriotic foci. We concluded that there were no similarities between EMs and OC for the cases included in our study, but we noticed differences in terms of Ki67 index and also between hormonal receptors expression in EMs associated with HGSOC, comparing with EMs associated with endometrioid OCs. These results may represent a "brick" for future researches on the less understood EMs associated with type II of OCs, especially with HGSOC. Identifying the best marker, which can predict the risk of developing OC for the patients with EMs, may lead to discover new specific therapeutic agents and, therefore, a better, tailored, therapy.