The authors calculated the volume of the prostate by transrectal ultrasonography and evaluated prostate-specific antigen (PSA) in 108 patients with benign prostatic disease or with clinically suspected carcinoma and in 35 normal subjects. In each case the PSA value was related to the corresponding gland volume (V), which gives a PSA/V index. 32 patients underwent transurethral resection, 23 underwent open prostatectomy and biopsy was performed in 53. Histological examination revealed benign prostatic hyperplasia in 63, prostatitis in 12 and carcinoma in 33. In normal subjects and in those with benign prostatic diseases, the mean PSA/V index was 0.090 and 0.099, respectively. In patients with prostatic carcinoma the ratio was 1.73. The authors propose that this ratio be used, as an alternative to the absolute value of PSA, to differentiate patients with benign and malignant diseases of the prostate.