I have a varicose vein on my left testicle? How serious is this?

Usually not serious. A varicose veins on your left testicle sound like a varicocele. These are usually benign in nature. The only difficulties arise the varicocele is very large, painful, or if fertility is affected. They can usually be left alone but if repair is needed it is usually done with microsurgery by a urologist or embolization by an interventional radiologist.
Not serious . While perhaps personally concerning, a varicose vein on the scrotum is only cosmetic unless very large and painful. Surgical removal is possible as an office procedure with very minimal discomfort.
In addition. Varicocele embolization has supplanted surgical techniques in terms of durability and failure rate. In addition, several studies have shown that coil embolization with sclerotherapy is more effective than gonadal vein embolization alone. Discuss with your urologist and/or vascular and interventional physician. Asymptomatic testicular varicoceles are usually not treated as others have discussed.