A variety of cell biological, pharmacological, crystallographic and clinical approaches have indicated that the antimitotic drug estramustine has interesting and unusual properties. Although designed as an alkylating agent, the marked stability of the carbamate linkage to the steroid carrier molecule prevents the formation of alkylating intermediates. The affinity of the parent molecule for microtubule associated proteins and the concomitant antimicrotubule activity have cytotoxic consequences in tumor cells. Both preclinical and clinical studies of estramustine in combination with other antimicrotubule agents have shown that this approach has great potential to achieve therapeutic advantage, especially in disease states such as hormone refractory prostate cancer.