The diagnosis of fetal erythroblastosis does not, as a rule, present serious difficulties to those familiar with its clinical picture and laboratory findings. However, in some instances the diagnosis is not easy. The difficulties are due to several reasons. One is that the term actually includes three distinct clinical entities: fetal hydrops, icterus gravis and congenital anemia of the newborn. The fact that they are related genetically and etiologically and that children of the same mothers may alternatingly manifest any one of the three forms of the disease does not lessen the diagnostic difficulties in individual cases. Another source of difficulty is that there are mild forms of the disease which may be overlooked or misinterpreted. The third difficulty is that none of the manifestations of the disease are pathognomonic. Every one of them may be found in certain other diseases. The diagnostic difficulties have been thrown into sharp focus