The human tumor stem cell assay (HTSCA) has been used to study the in vitro sensitivity rates of anticancer drugs used in the treatment of 115 patients with previously untreated and relapsing ovarian cancer. The data from these studies have identified patterns of cross resistance and residual sensitivity between these agents, and have allowed the prospective selection of single agents possessing in vitro activity for the treatment of 32 patients with relapsing disease. cis-Platinum and vinblastine were the most active agents in vitro against ovarian TCFUs from both previously untreated and relapsing patients. Prior therapy with even one drug was associated with the acquisition of resistance to several classes of compounds (e.g., melphalan resistance was almost always associated with in vitro adriamycin resistance, P less than 0.001). A clinical trial yielding similar data would have required nearly 450 evaluable ovarian cancer patients. In 11 of 32 patients in vitro testing predicted sensitivity to single agents: eight of these had partial remissions for a predictive accuracy of 73%. In 33 instances the HTSCA had 100% accuracy in predicting the lack of clinical response. Thus, the HTSCA for advanced ovarian cancer appears to have a similar predictive accuracy rate to the estrogen receptor assay for predicting the response to hormonal therapy for disseminated breast cancer.