A case of neonatal alloimmune thrombocytopenia with amegakaryocytosis is described. The baby was treated by double‐volume exchange transfusion with only a temporary benefit; 2 weeks of steroid treatment were unsuccessful but the platelet count did increase transiently after infusion of random donor platelets. The newborn recovered after intravenous immunoglobulin, with restoration of the megakaryocyte thrombocytopoiesis and platelet number. The rare antigen Ko^aon the paternal platelets and maternal immunization against it were detected.