There is a need for neonatal screening tools to improve the long-term clinical outcome of patients with primary immunodeficiency diseases (PID). Recently, a PCR-based screening method for both TRECs and KRECs using Guthrie card samples has been developed. However, the applicability of these excision circle assays is limited to patients with severe T or B cell lymphopenia (SCID, XLA and A-T), whereas the most common forms of PID are not detected. Absence of serum IgA is seen in a major fraction of patients with immunological defects. As serum IgA in newborns is considered to be of fetal origin, eluates from routinely collected dried blood spot samples might thus be suitable for identification of children with PID. To assess the applicability of such screening assays, stored Guthrie card samples were obtained from 47 patients with various forms of primary immunodeficiency diseases (SCID, XLA, A-T, HIGM and IgAD), 20 individuals with normal serum IgA levels born to IgA-deficient mothers and 51 matched healthy newborns. Surprisingly, normal serum IgA levels were found in all SCID, XLA, A-T and HIGM patients and, additionally, in all those IgAD patients born to IgA-sufficient mothers. Conversely, no serum IgA was found in any of the 16 IgAD patients born by IgA-deficient mothers. Moreover, half of the IgA-sufficient individuals born by IgA-deficient mothers also lacked IgA at birth whereas no IgA-deficient individuals were found among the controls. IgA in neonatal dried blood samples thus appears to be of both maternal and fetal origin and precludes its use as a reliable marker for neonatal screening of primary immunodeficiency diseases.