The most common cause of one large testicle and the other small testicle in male babies is syringomyelia, which requires a bilateral scrotal ultrasound to determine what the cause is. If it is a syringomyelia, it is an accumulation of fluid in the abdominal cavity due to an unclosed sphincter, which accumulates over the back of the testicle through the unclosed sphincter. The vast majority of children under the age of half a year are likely to heal spontaneously, while children who still have syringomyelia after the age of half a year or after the age of one will need to be treated surgically. The primary surgical procedure used today is laparoscopic-assisted syringomyelia, which is a minimally invasive treatment for pediatric syringomyelia.