Background Docetaxel (DOC) therapy is well tolerated and shows high response rates in patients with hormone refractory prostate cancer (HRPC). There are many reports on the effect of rapamycin (RPM) on the treatment of carcinogenesis. The goal of this study was to test whether RPM could enhance the susceptibility of both androgen-dependent and -independent prostate carcinoma cells to DOC.Methods Prostate cancer (PC) cell lines (LNCap, PC3 and AILNCap) were cultured and treated with RPM and DOC alone or in combination. The effects of therapeutic agents on cells were determined by the WST-1 assay. Apoptosis induction was confirmed by flow cytometric analysis. The apopcyto caspase colorimetric assay kit was applied to measure the activities of caspases 3 and 9. The antitumor effects of RPM and DOC against PC cells were also assessed in nude mice using four randomized groups: control, RPM, DOC and combination drug therapy by measuring tumor size. All the animals tolerated both RPM and DOC without significant weight loss.Results RPM and DOC caused dosage-dependent growth suppression of PC cells. RPM could increase the susceptibility of PC cells to DOC significantly, and combined treatment with RPM and DOC caused synergistic growth suppression in all examined PC cell lines by isobolographic analysis. Both RPM and DOC significantly induced apoptosis in a dosage-dependent manner. RPM (10 nmol/L), DOC (1 nmol/L), and combined treatment induced apoptosis rate were 8%, 17% and 38%, respectively (the control was 2%). RPM could promote the apoptosis induced by DOC in PC cell lines. Both RPM and DOC significantly increased the caspase activity in a dosage-dependent manner. The relative activities of caspase 9 in control, RPM, DOC and RPM+DOC groups were 0.22±0.02, 0.36±0.06, 0.47±0.05 and 0.84±0.08, respectively. The relative activities of cespase 3 were 0.21±0.02, 0.24±0.05, 0.42±0.06 and 0.81±0.09, respectively. Either RPM or DOC alone significantly inhibited the growth of PC cells in nude mice compared to the control. The combination of RPM and DOC produced a significant reduction in tumor volume when compared to RPM or DOC alone. After 5-week treatment, the tumor sizes of LNCap in control, RPM, DOC and RPM+DOC groups were (570±56) mm~3, (412±41) mm~3, (425±46) mm~3 and (221±26) mm~3, respectively.Conclusions RPM could significantly increase the susceptibility of both androgen-dependent and -independent PC cells to DOC; the synergy of RPM and DOC was demonstrated. RPM enhanced the DOC-induced upregulation of caspase activity, resulting in an increasing number of cells in sub-G1 phases. The synergy of the combined treatment might be observed in both androgen-dependent and -independent PC cell lines.