The aim of this study was to characterize the clinical and molecular markers of borderline serous ovarian tumors (BSOT), and to study their expression in the progression from benign lesions to advanced serous papillary ovarian carcinomas (SPOC). The clinical records of 20 patients with BSOT and 22 patients with SPOC were reviewed. Specimens from all these cases and from six benign ovarian serous cystadenomas were evaluated for expression of estrogen receptors (ER), progesterone receptors (PR), p53. HER-2/neu and Ki-67 by immunohistochemical techniques. The mean patient age and the age at menarche differed significantly between the compared groups of BSOT and SPOC (p=0.0006 and p=0.0014, respectively). No difference was observed comparing the other clinical parameters. The immunohistochemical analysis demonstrated a significant increase in the expression of ER (100% vs 72.7%), and a significant decrease in the immunoreactivity for p53 (0% vs 45.4%) and Ki-67 (2% vs 26.8%) in cases of BSOT compared with those of SPOC (p=0.007, p=0.0003 and p=0.012, respectively). No significant difference was demonstrated comparing the expression of PR and HER-2/neu. The immunostaining of benign ovarian serous cystadenoma specimens did not differ significantly from immunoreactivity observed in cases of BSOT. According to immunohistochemical analysis, BSOT had much more in common with benign serous tumors than with SPOC. The main difference between BSOT and SPOC was regarding the overexpression of p53 and Ki-67.