Testicular varcocele is the most common factor affecting male fertility. The determental effects of the varicocele on semen quality and fertility are codonsidered to be related to its severity. The aim of this study was to establish a quantitative radioisotopic method for the diagnosis and evaluation of testicular varicocele. 31 men attending our infertility clinic underwent scintigraphy with a gamma camera scanner (Apex 415) equipped with a pinhole low energy collimator (zoom factor 2). The evaluation was performed in the supine position and the collimator at 8.5 cm above the testicular area. The patients' red blood cells were labeled in-vivo by injection of SN-pyrophosphate before i.v. administration of To-99. An image of the scrotal area was obtained and the computer processing consisted of a drawing of the region of interest over the area of the varicocele, with background subtraction. The following indices were calculated: testes total count, varicocele area (in pixels) and average count per pixel (ACPP). The patients also underwent high resolution duplex sonography (HRDS) using a Multigon duplex scanner with a 7.5 Mhz transducer. Spermatic vein diameter, and reflux when present, were determined. HRDS was performed to compare and to validate the results obtained by scintigraphy. In men without varicocele mean ACPP was 2.80, in those with mild varicocele 3.76, in those with moderate varicocele 5.40 and in those with severe varicocele 7.48. A significant positive correlation was found between ACPP values and severity of the varicocele, as determined either by the diameter of the spermatic veins or by reflux of blood in the veins. We conclude that the ACPP index obtained by this new technique enables objective diagnosis and quantitative grading of testicular varicocele.