Severe combined immunodeficiency (SCID) is characterized by abnormal T and B cell function from birth. It is the severest of all congenital immunodeficiencies and unless immunologic reconstitution is achieved through bone marrow transplantation or enzyme replacement, death usually occurs by 2 years of age. Several types of SCID have been identified and clinically classified as T-B+ or T-B- SCID depending on affection of B cells. T cells and subsets are low to absent in all types. X linked SCID is the commonest form of SCID and patients present as T-B+ NK- SCID (decreased T cells, normal B cell quantity and low Non-Killer cells) as was seen in our patient. Males are affected. Affected infants present with frequent episodes of diarrhea, pneumonia, sepsis and cutaneous infections within first few months of life. Failure to thrive after infections is common. Viral infections such as Varicella, Measles, Parainfluenzae, CMV, Epstein Barr Virus and fungal infections such as Candida, Pneumocystis Carinii (PCP) are common. Our patient had a candidial urinary tract infection. BCG vaccine can lead to disseminated TB. Infants lack the ability to reject foreign tissue and hence are at risk for graft-versus-host disease from maternal immunocompetent T cells or from T cells in non-irradiated blood transfusion or allogenic bone marrow transplant. Thymus tissue is hypoplastic and hypoplasia of adenoids, tonsils and peripheral lymph nodes is seen as was seen in our patient. Investigations reveal profound lymphopenia with diminished serum immunoglobulins and no antibody formation following investigation. Antibody levels may initially be normal due to passively transferred maternal antibodies. Analysis of lymphocyte subpopulation helps to identify the type of SCID. Treatment consists of bone marrow transplant. Gene therapy with enzyme replacement is useful in ADA deficient SCID.