Once phenylketonuria is diagnosed, immediate treatment is necessary; the younger the age at which treatment is started, the better the prognosis. The main approach is to use a low phenylalanine formula milk for treatment. Once the blood phenylalanine concentration has decreased to the desired level, we can gradually begin adding small amounts of natural diet. For small infants, breast milk is preferred since it contains much less phenylalanine than cow's milk. For older infants, additions such as cow's milk, porridge, noodles, and eggs are acceptable, but supplements should still follow the principle of being low in protein and phenylalanine. The amount and frequency of these additions should be determined based on the phenylalanine concentration, as too high or too low levels can affect growth and development. This condition can only be managed by controlling symptoms and maintaining blood phenylalanine levels. Long-term treatment is required; it cannot be cured, only controlled.