Epididymitis: This is often the most common cause of unilateral testicular swelling, especially in sexually active young men. It is usually caused by a bacterial infection, such as Chlamydia or Gonorrhea, and presents with pain, swelling, and tenderness of the epididymis.

Orchitis: Inflammation of the testis, often due to viral infections like mumps, can cause unilateral testicular swelling. It presents with testicular pain and swelling.
Hydrocele: A collection of fluid around a testicle, leading to swelling of the scrotum, which can be unilateral. It is usually painless.
Varicocele: An enlargement of the veins within the scrotum, similar to varicose veins, which can cause a feeling of swelling or a "bag of worms" sensation.
Testicular torsion (early stages): Although more commonly associated with acute severe pain, early stages might present with swelling before the pain becomes severe.

Testicular Torsion: A medical emergency that requires immediate attention. It involves the twisting of the spermatic cord, cutting off the blood supply to the testicle. Prompt diagnosis and treatment are crucial to save the affected testicle.
Testicular Cancer: Although less common, it is critical to consider, especially in men between 15 and 35 years old. Swelling can be a presenting symptom, along with a palpable mass.

Spermatocele: A cystic structure filled with sperm, which can cause swelling but is usually painless and less common.
Cystic dysplasia of the testis: A rare congenital anomaly that can cause testicular swelling.
Henoch-Schönlein Purpura (HSP): A form of vasculitis that can rarely cause testicular swelling among its many manifestations.
Filariasis: In endemic areas, this parasitic infection can cause hydrocele and, less commonly, other forms of testicular swelling.