Under normal circumstances both testicles should be roughly equal, if they appear to be significantly different, it is recommended to go to the hospital for further examination to clarify the possible causes. First, if epididymitis or orchitis occurs in a unilateral testicle, two testicles can appear to be different. The testicle on the affected side may have an enlarged volume or a restricted mass, accompanied by obvious pain, redness and swelling of the scrotal skin, and further ultrasound or blood tests are needed to clarify. Secondly, if there is unilateral testicular syringomyelia, there can be unequal volume of both testicles. Generally, it is not accompanied by obvious pain, but the larger size can affect life or urination, and ultrasound examination of the scrotum is needed to confirm the diagnosis. Thirdly, if there is unilateral testicular malignancy, testicular cancer can also appear when the testicles on both sides are different, mainly manifesting as different density of testicles, with the testicles on the diseased side increasing in density and hard texture when touched, while the testicles on the normal side are softer when touched. In the early stage, there may be slight pain, and in the later stage, after the volume increases, there may be obvious painful downward movement, and testicular tumor index examination and testicular CT examination are needed to confirm the diagnosis.