The biological mechanisms involved in androgen-dependent and -independent prostate cancer growth after castration were analyzed in the LuCaP 23.1 human prostate cancer xenograft model. Athymic mice (n = 82) bearing LuCaP 23.1 xenograft were castrated and tumors were harvested at different time points from day 0 to day 112 post castration. In each group of mice, tumor growth rate (TGR), serum PSA concentration, percentage of tumor cells incorporating bromodeoxyuridine (BUdR index), percentage of apoptotic tumor cells assessed by morphological analysis (apoptotic index), and presence of apoptosis-related DNA "ladder" were analyzed. Castration induced a significant decrease in TGR and serum PSA from day 1 to day 7, and a progressive increase in the 2 parameters from day 14 to day 112, heralding androgen-independent tumor relapse. Meanwhile the BUdR and apoptotic indexes varied as follows after castration: an increase was noted for both at day 3, a significant increase in apoptotic index with a decrease in BUdR index from day 5 to day 14, and a progressive decrease in apoptotic index while BUdR index remained at 50% of the pre-castration value from day 28 to day 112. DNA ladder was present sparsely in tumors grown in non-castrated hosts, universally present in tumors from day 1 to day 28 post castration, and frequent in tumors from day 56 to 112. Castration-induced effects in LuCaP 23.1 tumors were characterized by an increase in number of apoptotic cells and a decrease in proliferative activity. The androgen-independent tumor relapse after castration was associated with a low apoptotic index with no increase in proliferative activity.