We herein report five new cases of severe combined immunodeficiency with hypereosinophilia, the so-called familial reticuloendotheliosis first described by Omenn. It is characterized by erythroderma, polyadenopathy, hepatosplenomegaly, severe and repeated infections, protracted diarrhoea with failure to thrive. There is marked eosinophilia as well as a profound immunodeficiency. The immunologic abnormalities consist of an increase in T cell number, a B cell lymphopenia and a complete lack of humoral and cellular immune responses to antigens. A deficiency of lymphocytes 5'-nucleotidase has been inconstantly found. Histologic findings are characteristic, consisting of severe T and B lymphocyte depletion in lymphoid organs with infiltration by histiocytes and, to a lesser extent, eosinophils. The outcome was uniformly fatal within the first year of life. Treatment by a combination of parenteral nutrition, steroids and epipodophyllotoxin was effective in obtaining the complete remission of clinical manifestations due to the histiocytic and eosinophilic infiltration in two patients. However, the treatment failed to correct the immunologic defect. These results indicate that the histiocytic infiltration is possibly not responsible for the immunologic detect observed in this condition.