SummaryDescription of a case of erythroleukaemia (morbus Di Guglielmo) in an infant of 7 months of age, where the symptoms manifested themselves shortly after smallpox vaccination. The patient had high fever, marked hepato‐splenomegaly, anemia and thrombocytopenia as well as leucocytosis, with the occurrence of young, often atypical myeloid and erythroid elements in the blood. The erythroid elements dominated over the myeloid elements in the marrow. The treatment, consisting of aminopterin, supported by blood transfusions and antibiotics, did not influence the acute fatal development, although the young myeloid elements (myeloblasts, promyelocytes, and myelocytes) soon disappeared from the blood, and the liver and spleen grew smaller. At autopsy, erythroid and myeloid proliferation were found in the marrow, liver, spleen, lymphnodes, thymus and intestine