Background: Inborn errors of metabolism (IEMs) make up a large group of rare disorders caused by an inherited deficiency or absence of proteins that have enzymatic, carrier, receptor, or structural roles. This study is aimed to early detection, diagnosis, and intervention to improve outcome. Patients and methods: This cross-sectional study was done in Pediatrics Department, Zagazig University Hospitals. It included 65 cases with suspected inborn errors of metabolism. All the studied cases were subjected to full history taking, clinical examination and Laboratory investigations. Results: The age of neonates, infant and children were 35 (54.6%), 22 (34.37%), 8 (10.93%) cases respectively, 34 cases were females and 30 males. The most common complaint was poor suckling in 14 cases (21.8%) then lethargy as well as fever in 6 cases (9.37%). Disturbed consciousness and hepatosplenomegaly were present in 11 cases (17.18%). Metabolic disorders including metabolic acidosis (25%), hypoglycemia (3.12%), raise lactate level (4.68%), hyperammonemia (14) and presence of ketonuria in (4.68%) of cases, respectively. The different disease types of the studied cases after routine and specific laboratory finding, our results showed that, 18 cases Suspected IEM (not diagnosed), 11 cases Diagnosed metabolic (non PKU), 36 cases Diagnosed Phenylketonuria (PKU). Conclusion: Early diagnosis of inborn errors of metabolism (IEMs) during neonatal period or infancy for starting its proper treatment will improve outcomes, control complications of metabolic diseases and decrease the mortality rates.