Four cases of advanced stage (II or III) and one case of early stage (IC) borderline malignant serous cystadenocarcinomas of the ovary were maintained on culture dishes coated with an extracellular matrix (ECM) produced by bovine corneal endothelial cells. Cells harvested for chromosomal analysis after 2-3 days showed diploid or near-diploid modalities in all cases. Banded chromosome studies in two cases revealed nonrandom clonal abnormalities with trisomy 2, 7, and 12 in seven of 13 metaphases. No structural abnormalities were noted. These cytogenetic findings differ from those found in malignant serous tumors of the ovary. In addition, borderline tumor cells digested the ECM in all cases and formed a cribiform pattern within a few days of primary culture. This study suggests clonal progression from early to advanced stages of borderline malignant serous tumors; readily distinguishable from overtly malignant serous tumors of the ovary. Ability of tumor cells derived from both primary tumors and metastatic implants to digest the ECM implies the possibility that borderline serous tumors have invasive potential.