Although prostate cancer (PCa) is the most frequently diagnosed cancer in males, little is known about the mechanisms involved in its progression. Recent in vitro studies suggest that coactivators of the androgen receptor play an important role in PCa progression. We have shown previously that p300 is involved in androgen receptor transactivation. In the present work, we studied 95 patients with biopsy-proven PCa who underwent prostatectomy as treatment of their tumors between 1995 and 1998. We found that p300 correlated with in vivo proliferation (P = 0.009) as determined by MIB-I expression. Moreover, high levels of p300 in biopsies predicted larger tumor volumes (P < 0.001), extraprostatic extension (P = 0.003), and seminal vesicle involvement (P = 0.002) at prostatectomy, as well as PCa progression after surgery (P = 0.01). Furthermore, we found that the disruption of p300 transcripts through small interfering RNA inhibited PCa cell proliferation both at the basal level and on interleukin 6 stimulation. We conclude that p300 plays an important role in PCa cell proliferation, as well as PCa progression.