If the testicles are large on one side and small on the other, and accompanied by swelling and pain, the possible causes are: 1. Acute orchitis: it can be accompanied by an increase in the volume and swelling of the testicles on the inflammatory side, and accompanied by a feeling of testicular drop, and the pain is aggravated when moving, and can be relieved when resting. Severe patients can be accompanied by symptoms of fever, ultrasound examination can clearly diagnose whether or not suffering from orchitis; 2. Acute epididymitis: mostly caused by urinary tract infection or seminal tract infection, manifested as an increase in the volume of the epididymis, involving the testicles can appear testicular enlargement, the appearance of the testicles on one side of the large, one side of the small, accompanied by obvious pain. In the acute stage of inflammation, the testis and epididymis are closely adhered to each other, so it is often difficult to distinguish them, and ultrasound examination is needed to identify them. 3. CT examination of testis and testicular tumor indicators are needed to further rule out whether it is caused by testicular tumor.