A group of eight children with ataxia-tengiectasia (A-T) who presented with serum Ig levels suggestive of hyper-IgM syndrome (HIGM) are reported from the Department of Paediatrics, Reinier de Graaf Gasthuis, Delft, and other centers in the Netherlands, Iceland, and Belgium. All patients ultimately showed clinical features of A-T (ataxia, telangiectasia, recurrent respiratory tract infection), and all had raised serum a-fetoprotein levels; in 7 the diagnosis of A-T was confirmed by DNA analysis. 
COMMENT. Hyper-IgM syndrome is a primary immunodeficiency disease that presents with recurrent infections, decreased serum immunoglobulin IgG and IgA and normal or raised IgM levels. Ataxia-telangiectasia also presents with recurrent infections, and at an early age the ataxia and telangiectasia may be absent. Immunodeficiency in A-T is associated with decreased serum IgG and/or IgA levels, and 10% of cases have normal or raised IgM levels. In the absence of typical clinical signs of A-T, the diagnosis may be confused with hyper-IgM syndrome. Elevated a-fetoprotein and low T lymphocytes distinguish A-T cases.