Since 1989 we have used serum prostate specific antigen (PSA) levels as an indication for ultrasound guided systematic biopsies of the prostate. Realizing that the PSA level in part reflects prostatic glandular epithelial volume, we reviewed the accumulated data on our last 2,340 biopsies to determine if the quotient of PSA and prostatic volume, prostate specific antigen density, provided any further diagnostic information. There were evaluable data for 2,020 patients. Prostate specific antigen density levels are shown to have a strong correlation with the diagnosis of prostate cancer and provide a more reliable indication for ultrasound guided biopsy of the prostate than PSA alone.