The testicles of children are large on one side and small on the other, and the possible causes are as follows: 1. Testicular syringomyelia. Testicular syringomyelia is mostly congenital, that is, there is a large amount of liquid around the testicle, the liquid wrapped around the testicle appears to be relatively large testicular volume, there is no obvious pain, and it does not affect the activities of the child, sometimes the testicle volume can be slightly reduced after lying down. It is recommended that an ultrasound examination be performed to confirm whether the testicle is suffering from syringomyelia. 2. Acute and chronic epididymitis or orchitis. Secondary orchitis is common in children after mumps virus, and the testicles on the affected side may be enlarged and painful. It is recommended that relevant tests, such as ultrasound and mumps virus tests, be performed to rule out this condition. If the diagnosis is secondary orchitis caused by mumps, hospitalization is recommended for antiviral treatment to avoid damage to testicular function. 3. varicocele. The volume of the testicle on the varicose side is also larger than that on the normal side, because the varicose blood vessels are attached around the testicle and appear to have slightly increased volume. If there is no obvious pain in this case, ultrasound examination of the scrotum, testicles and spermatic cord is recommended to rule it out.