The objective of this study is to compare progression-free survival (PFS), based upon investigator assessment using Response Evaluation Criteria In Solid Tumors version 1.0 (RECIST 1.0) and clinical findings, in participants with platinum-resistant ovarian cancer who receive combination therapy with vintafolide and pegylated liposomal doxorubicin (PLD/Doxil®/Caelyx®) with that in subjects with platinum-resistant ovarian cancer who receive PLD alone.