The human tumor cloning assay (HTCA) has been available for preclinical and clinical applications for the last 11 years. This article examines the usefulness of that assay both in the practice of clinical oncology and in the development of new antineoplastic agents. In the area of prediction of response of an individual patient's tumor to a particular antineoplastic agent, in a total of 2274 correlations in a variety of clinical trials, the assay has shown a remarkably good ability to predict whether a patient's tumor would respond to a particular agent (percent true positives 69%; percent true negatives 91%). However, despite this ability to predict these responses, the assay has not yet been accepted for general clinical use, because there have not been rigorous trials to prove the assay will improve patient survival. These rigorous clinical trials are now underway. In the area of drug development, the HTCA has been used to screen for new antineoplastic agents as well as to pinpoint tumor types against which the new agent will be active in phase II clinical trials. As will be seen in this review, the HTCA has been most successful in this area.