Ataxia-telangiectasia (A-T) is characterised by progressive neurological abnormalities, oculocutaneous telangiectasias and immunodeficiency (decreased serum immunoglobulin (Ig)G subclass and/or IgA levels and lymphopaenia). However, 10% of A-T patients present with decreased serum IgG and IgA with normal or elevated IgM levels. As cerebellar ataxia and oculocutaneous telangiectasias are not present at very young age, these patients are often erroneously diagnosed as hyper IgM syndrome (HIGM). We present eight A-T patients showing serum Ig levels suggestive of HIGM upon first presentation. All patients had decreased numbers of T lymphocytes, unusual in HIGM. The diagnosis A-T was confirmed by demonstrating elevated alpha-fetoprotein levels in all patients. To prevent mistaking A-T patients for HIGM we propose to add DNA repair disorders as a possible cause of HIGM.