Long misclassified as an alkylating agent, estramustine phosphate is a stable conjugate of estradiol and nornitrogen mustard with antimitotic properties. Binding of the drug to microtubule-associated proteins, tubulin, and proteins of the nuclear matrix are presently considered to be the most likely mechanisms underlying the cytotoxicity of estramustine in androgen-independent prostatic carcinoma. Identification of these mechanisms of action has led to clinical reevaluation of estramustine phosphate (EMP) in several rationally designed drug combinations. Combination of EMP with either vinblastine, paclitaxel or etoposide has produced antitumor responses in 30% to 60% of patients with metastatic hormone-refractory prostate carcinoma as determined by reduction in bidimensionally measurable soft tissue disease, pain, and serum prostate-specific antigen. Whereas the antitumor activity of the combinations has been greater than additive for the single agents, the toxicities of treatment have not been greater than predicted for the individual drugs. The promising results of EMP-based chemotherapy encourage additional laboratory and clinical investigations to develop more effective therapy for hormone-refractory disease and for selected patients with earlier stages of prostate cancer.