In a recent publication, we have shown that delphinidin, an anthocyanidin induces apoptosis and cell cycle arrest in highly metastatic human prostate cancer (PCa) PC3 cells. Extending these studies, we provide additional evidence that delphinidin induces apoptosis and cell cycle arrest in androgen refractory human PCa 22Rnu1 cells and that these effects are concomitant with inhibition of NFkappaB. We observed that delphinidin treatment to 22Rnu1 cells resulted in a dose-dependent (i) G(2)/M phase cell cycle arrest, (ii) induction of apoptosis (iii) and inhibition of NFkappaB signaling. The induction of apoptosis by delphinidin was mediated via activation of caspases since a general caspase inhibitor Z-VAD-FMK significantly reversed this effect. Delphinidin treatment to cells resulted in a dose-dependent decrease in (i) phosphorylation of IKKgamma (NEMO), (ii) phosphorylation of NFkappaB inhibitory protein IkappaBalpha, (iii) phosphorylation of NFkappaB/p65 at Ser(536) and NFkappaB/p50 at Ser529, (iv) NFkappaB/p65 nuclear translocation, and (v) NFkappaB DNA binding activity. Taken together, our data show that delphinidin induces apoptosis of both androgen independent and androgen refractory human PCa cells via activation of caspases and in addition, this effect might be due to inhibition of NFkappaB signaling. We suggest that delphinidin could be developed as a novel agent against PCa.