The main concern with soy is due to the phytoestrogens (aka isoflavones) that are naturally present in soy. They are said to have an "estrogenic effect" because they're similar in structure to the hormone estrogen. Babies who drink soy formula will obviously be consuming much larger quantities of these isoflavones than babies who don't drink soy formula. So it's natural to wonder whether these isoflavones might have a negative hormonal impact on babies who are consuming so much during their most critical development period. There has been some research done on this, and the conclusion was that soy formula during infancy didn't have a discernible impact on babies' hormone levels later in life. The American Academy of Pediatrics also says that soy formula is perfectly fine for babies with a milk allergy or lactose intolerance. The safety and health of soy formula have been studied extensively and it appears to be safe from a neuro-developmental perspective.  Additionally, there are no clear negative effects regarding an infant’s sexual development and growth. However, it is always a good idea to check with your pediatrician before starting any new formula, including soy based formulas. My main issue with soy is that most soy is genetically modified, so if you do decide to choose a soy formula, make sure you're choosing an organic formula to ensure the soy is non-GMO!