Shorter androgen receptor gene CAG repeat length has been associated with an increased risk of prostate cancer, an earlier age of onset, and more advanced stage of disease. Studies comparing the distribution of CAG repeat lengths within different populations have reported that racial groups with higher prostate cancer incidence also have shorter CAG repeat lengths. We evaluated CAG repeat length in 685 black men in Louisiana, South Carolina, and the District of Columbia who were participating in prostate cancer screening, comparing the 118 who were diagnosed with prostate cancer with 567 who had normal serum prostate-specific antigen levels and no evidence of cancer on digital rectal examination. The median CAG repeat length was 21 among cases and 19 among controls (P = 0.11). Cases were significantly older than controls, with a median age of 68 years compared with 54 years (P < 0.0001). After adjusting for age, we found no association between prostate cancer risk and CAG repeat length (odds ratio, 1.05; 95% CI, 0.98-1.13; P = 0.16). Dividing CAG repeat lengths into septiles and calculating the odds ratio for each revealed no specific repeat-length range with a significantly elevated or depressed risk of prostate cancer, but a trend test showed a significant association between longer CAG repeat lengths and an elevated risk of prostate cancer (P = 0.02). Neither grade nor stage was associated with CAG repeat length. This study confirms earlier reports that black men have shorter CAG repeat lengths than reported white and Asian populations. We did not find an increased risk of prostate cancer among black men with fewer CAG repeats.