Inborn errors of metabolism are inherited defects in human metabolism. Many different metabolic diseases are known and, even though individually rare, collectively they are common and may appear in any pediatric department from time to time. To improve the prognosis for metabolic diseases, early recognition is necessary. Many infants with metabolic diseases can be diagnosed with routine biochemical tests and metabolic screening of urine. For some metabolic diseases, an early diagnosis will lead to specific treatment and improved prognosis, for others to genetic counseling and prenatal diagnosis. To achieve this it is important to think metabolic and screen for metabolic diseases when examining for sepsis. The article reviews the principles of early diagnosis and treatment of metabolic diseases in the first year of life.