The human tumor stem cell assay (HTSCA) provides a means of performing drug sensitivity measurements on human tumor cells in primary culture. Results from such assays offer potential for improving cancer chemotherapy by identifying drugs useful for treatment of individual patients' tumors and through application to screening new compounds for antitumor activity. While existing data supports the potential of the assay in both areas, the assay also poses significant drawbacks. Many of these drawbacks relate to technical aspects of the assay and can be eliminated or reduced by further assay development. In this paper, we describe some of the technical drawbacks in detail and some approaches which have been successful in minimizing them. Continued advances in this area should make it possible to more fully realize the potential of the human tumor stem cell assay.