The purpose of this article is to demonstrate how CA-125 could be used in clinical trials to ascertain the efficacy of new drugs for ovarian cancer. Studies that have investigated the use of CA-125 in assessing response and progression of ovarian cancer are reviewed. A precise CA-125 response definition that requires either a 50% or 75% decrease in CA-125 levels has been shown in trials of both initial chemotherapy and in phase II trials to predict accurately the response in comparison with standard response criteria. A simpler response definition that is based on just a 50% decrease in CA-125 levels has been proposed by the Gynaecological Cancer Intergroup (GCIG) but requires further validation. Definitions for progression have also been proposed by the GCIG on the basis of a confirmed doubling of CA-125 levels from either the upper limit of normal or the nadir CA-125 level. These CA-125 definitions for progression falsely predict progression in fewer than 2% of patients and can be used to define the date of progression. Precise definitions for response and progression according to CA-125 should be incorporated into ovarian cancer clinical trial protocols. These definitions already have been shown to be valuable in assessing efficacy of new agents but require further prospective evaluation.