Epididymitis: This is often the most common cause of bilateral testicular swelling, especially in sexually active men. It is an inflammation of the epididymis, usually caused by bacterial infection, which can lead to swelling, pain, and tenderness in the testicles.

Orchitis: Inflammation of one or both testicles, usually caused by viral or bacterial infections. It can present with testicular swelling, pain, and may be accompanied by epididymitis.
Hydrocele: A condition characterized by the accumulation of fluid around a testicle, leading to swelling. It can be primary (idiopathic) or secondary due to other conditions like injury, infection, or tumor.
Varicocele: Enlargement of the veins within the scrotum, similar to varicose veins. While it typically presents unilaterally, it can occasionally cause bilateral testicular swelling due to the backflow of blood.

Testicular Torsion: A medical emergency where the spermatic cord becomes twisted, cutting off the blood supply to the testicle. It requires immediate attention to save the affected testicle. While it usually presents acutely and unilaterally, any sudden onset of testicular swelling or pain warrants consideration of this diagnosis.
Testicular Cancer: Although less common, testicular cancer can present with testicular swelling. Given its potential for serious outcomes if not treated early, it is crucial not to miss this diagnosis.

Henoch-Schönlein Purpura (HSP): A form of vasculitis that can affect various parts of the body, including the testicles, leading to swelling and pain. It is more common in children but can occur in adults.
Sarcoidosis: A condition that can cause inflammation in various parts of the body, including the testicles, leading to swelling. It is relatively rare and usually presents with other systemic symptoms.
Filariasis: In areas where it is endemic, filariasis can cause hydrocele and, less commonly, epididymitis, leading to testicular swelling. It is due to a parasitic infection transmitted by mosquitoes.