The authors report a case of neonatal haemolytic disease (HDN) attributable to maternal allo immunization to a high frequency antigen present on paternal red blood cells which have been identified as Colton-a (Coa). The case seemed worthy of reporting not only for the relative rarity of HDN from anti Coa but for the rigorous approach followed in the immunohaematological follow-up of pregnancy and for the diagnostic effort expended in identifying the specificity of the antibody. A family study of ascendants, collaterals and descendants was also carried out using genotyping techniques.