Inborn errors of metabolism are a large group of rare but often serious metabolic disorders that generally arise from deficiency or malfunction of enzymes that regulate conversion of various substances into others. Medical problems arise due to accumulation of toxic intermediates or decreased synthesis of essential compounds.

Common examples include phenylketonuria and Gaucher's disease.

Inborn errors may be suspected in children with poor development, seizures or characteristic features. Diagnosis is usually by mass spectrometry of serum and urine, or by DNA testing.