Varicocele is a mass of enlarged and dilated veins that develops in the spermatic cord within the scrotal sac. The spermatic cord is made up of veins, arteries, lymphatic vessels, nerves, and the duct that carries sperm to the seminal vesicles from the testes. If the valves that regulate bloodflow from these veins are congenitally defective, blood does not circulate from the testicles efficiently, causing swelling in the veins above and behind the testicles with resulting warming of the testes.

A varicocele can develop in one testicle or both, but in about 85% of cases it develops in the left testicle. The left spermatic vein drains into the left renal vein, which transits between the superior mesenteric artery and the aorta; it is theorized that these two arteries can compress the left renal vein and thus impede bloodflow from the spermatic vein, and in the presence of defective venous valves, cause increased distal backpressure and dilation, resulting in formation of a varicocele.