Varicoceles are a tangled network of blood vessels or swelling of the veins on the testicles. It is similar to varicose veins in the legs but in this condition a vein called the gonadal vein (see diagram) has weak defective valves resulting in the blood pooling in varicose veins in the scrotum. It is a relatively common condition (affecting approximately 10 percent of men) that tends to occur in young men, usually during the second or third decade of life. Sometimes, varicoceles cause no symptoms and are harmless. But sometimes a varicocele causes pain, testicular atrophy (shrinkage), or fertility problems.