This article describes the clinical signs, abnormal chemistry, histopathology, and biochemical tests that assist in the diagnosis of many inherited metabolic diseases. Small molecule diseases (for example, amino acid disorders, organic acidurias, and galactosemia) may present with acute illness and lead to unexplained death. Diagnosis of the specific enzyme defect may permit treatment by dietary changes or vitamin supplementation. Large molecule diseases are degenerative disorders due to storage of glycogen, glycoproteins, or mucopolysaccharides. Table 7 presents a compendium of laboratory investigations one should consider using when abnormalities are found in multiple organ systems. The investigation of a child with a mucopolysaccharidosis is used as an example of the use of this table. Investigating pediatricians should realize that many laboratory tests involved in the diagnosis of metabolic disease required special collection and handling by the laboratory.