Summary—Fresh hyperplastic prostate glands from 19 patients who underwent open prostatectomy were weighed and their volume measured. After a simulated transurethral resection the weight and the volume of the pieces were determined. The volume of each enucleated specimen was plotted against the volume after electroresection. The regression coefficient was 0.8039. Similarly, the regression coefficient for the weight was 0.7864. Therefore a diminution in volume and weight of approximately 20% can be expected after electroresection. The specific weight close to one remained unchanged. The supracollicular distance was an unreliable indicator of the size of the prostat