After initial regression in response to androgen deprivation, most prostate cancers develop resistance to endocrine therapy. Identification of cellular and molecular changes occurring during endocrine therapy-induced regression and subsequent hormone insensitivity may point to mechanisms underlying the transition to hormone-independent prostate cancer. A series of untreated (n = 24), regressed (n = 15), and endocrine therapy-resistant (n = 10) prostatic adenocarcinomas were analyzed using immunohistochemistry with regard to cytokeratin 5 and 18, androgen receptor (AR), and epidermal growth factor receptor (EGF-R) expression in tumor cells. Using semiquantitative reverse transcription-polymerase chain reaction, the amount of AR mRNA also was determined. In regressed and therapy-resistant prostate cancers, an increase in cytokeratin 5-positive tumor cells was noted when compared with untreated carcinomas. Similarly, the proportion of EGF-R-positive tumor cells increased in the treated cases, whereas the proportion of AR-positive tumor cells dropped in regressed carcinomas and increased in hormone-refractory cancers. In the latter group, an eightfold higher level of AR mRNA was observed when compared with the other cases. Changes in the proportion of cytokeratin 5 and EGF-R-positive tumor cells suggests that during androgen deprivation an enlarged subpopulation of tumor cells with combined features of basal and secretory phenotypes arises. The increased proportion of AR-positive tumor cells during the transition from the regression phase to the hormone escape phase points to an important role of AR overexpression in this process.