Early/late onset sepsis (EOS/LOS) and necrotizing enterocolitis (NEC) are common diseases during the neonatal period, with poor prognosis in severe cases. The early clinical manifestations are not specific, the diagnosis is difficult, currently-used non-specific laboratory tests (C-reactive protein, procalcitonin, etc.) have disadvantages, which may lead to missed diagnosis or misdiagnosis. Serum amyloid A (SAA) is a novel acute phase reactant that increases significantly in the early stage of EOS/LOS and NEC, and lasts for a long time, it is related to the severity of the disease and can reflect the therapeutic effect, thus it can be used as a biomarker for diagnosis and treatment of these two diseases. This paper reviews SAA and its value in the early diagnosis of EOS/LOS and NEC, providing new references for the diagnosis and treatment of such diseases.