Purpose: Black American men experience disproportionate mortality from prostate cancer (CaP) compared with white American men. Differences in outcome may stem from differences within the androgen axis. Since serum testosterone levels appear to be similar by race in men with CaP, we measured and compared androgen receptor (AR) protein expression in malignant and benign prostate tissue from black and white men who underwent radical prostatectomy for clinically localized CaP. Materials and Methods: Archived radical prostatectomy specimens obtained from 25 white and 25 black men had AR protein antigen retrieved and immunostained. AR protein expression from CaP and benign tissue was assessed by 2 methods. Automated digital color video image analysis was used to measure the percent area immunostained for AR protein and the intensity of expression (mean optical density). Visual scoring was performed to compare results with automated values. Results: In black compared with white men malignant nuclei were 27% more likely to immunostain for AR (p = 0.005) and in immunopositive nuclei AR protein expression was 81% greater (P = 0.002). Visual scoring of malignant nuclei revealed that AR immunostaining was significantly increased in black vs white men (171 +/- 40 vs 149 +/- 37, p = 0.048). In immunopositive benign nuclei AR protein expression was 22% greater in black than in white men (p = 0.027). Visual scoring of benign nuclei revealed 20% increased immunostaining in black vs white men, although this difference did not attain statistical significance (p = 0.065). Racial differences in AR protein expression were not explained by age, pathological grade or stage, although serum prostate specific antigen levels were higher in black men (9.7 +/- 7.5 vs 15.5 +/- 12.2 ng/ml, p = 0.049). Conclusions: AR protein expression was 22% higher in the benign prostate and 81% higher in the CaP of black African compared with white men. CaP may occur at a younger age and progress more rapidly in black than in white men due to racial differences in androgenic stimulation of the prostate.