Neoadjuvant androgen suppression (NAS) can reduce the number of tumor clonogens prior to radiation, thus increasing the tumor control probability. Also, NAS may sensitize tumor cells to radiation if cell kill by both modalities follows a common pathway. The timing and sequence of NAS and radiation are important, with radiation being most effective if given at the point of maximal tumor regression. The biologic rationale for NAS + radiation has been reinforced by results from randomized trials, in particular RTOG 8610. However, many murine adenocarcinomas respond to androgen deprivation by a reduction in the proliferation rate and arrest in G(0), and in vitro data suggest that this arrest may interfere with radiation-induced cell killing. The mechanism of cell killing after low-dose-rate radiation (brachytherapy) may be different from that after high-dose-rate treatment. There are no reported experimental data assessing the interaction of NAS and brachytherapy to determine whether the combination offers a theoretical advantage or is potentially deleterious. Whether we understand the mechanism or not, clinical trials seem to support a positive interaction of NAS with external-beam radiation, but we have only begun to explore the timing and sequence that will provide the maximal effect. It cannot be assumed that the same advantage will hold with brachytherapy.