Because prompt intervention may prevent complications, early diagnosis is important in many inherited metabolic diseases. Early diagnosis of Severe Combined Immunodeficiency (SCID) is critical - because chances for successful treatment are highest for infants who have not yet experienced severe opportunistic infections. SCID is a rare disease that can be detected in newborn infants (i.e., those more or equal 1 month of age) by automated blood count and manual differential. Early diagnosis of SCID is rare since, because estimates of the incidence rate range from one in 50,000 to 100,000 births, most pediatricians do not routinely count white blood cells in newborns. Tests for T-cell lymphopenia (TCLP) using dried blood spots (DBS) could be used to identify children with SCID - as well as for other immunodeficiencies that would not be apparent until after the child developed an infection. Screening newborns for SCID would allow early diagnosis and treatment -- as well as genetic counseling for the family.