This study of 66 children with congenital heart disease found 26 (39%) with IgG subclass deficiency, the majority being of the IgG4isotype. Conventional immunoiogical assessment (IgG, IgA, IgM, T cell) revealed 21 (32%) with immunodeficiency, while inclusion of IgG subclass assessment revealed a total of 35 (53%) of the 66 children had immuno‐deficiency. Children with conotruncal lesions appeared to be predisposed to immunodeficiency affecting T cells and IgG subclasses (especially IgG4) while those with shunt and stenotic lesions had a broad spectrum of immunoglobulin deficiencies. There was significant correlation between immunodeficiency and proneness to infection in these children (p<0.01). These results suggest that immunodeficiency is a frequent occurrence in children with congenital heart disease, and that IgG subclass measurements should be added to the diagnostic work