If the testicles suddenly swell, it is recommended to go to the hospital for further examination to clarify the cause. The common causes are: 1. When testicular infection occurs, there can be an acute increase in the overall size of the testicle, and it is accompanied by obvious pain, causing changes such as redness and swelling of the scrotal skin in severe cases. In men, this is often seen as a result of mumps virus infection, which causes orchitis, or secondary to urinary tract infection. 2. Epididymitis. The epididymis is attached to the posterior part of the testicle. When urinary tract or seminal tract infection occurs, it first shows changes such as infection of the epididymis, increase in size, and swelling caused by the involvement of the testicle. Ultrasound examination is needed to identify this. 3. When infected testicular syringomyelia occurs, sudden enlargement of the testicle may also occur. In this case, the swelling is more obvious in appearance, but the pain is relatively mild, and when touched by hand, the boundaries of the testicle are not clear to touch, and ultrasound examination can identify whether it is effusion or testicular swelling.