An unusual form of beta thalassemia is described in two children of unrelated families. Its main features are a severe anaemia with a low reticulocyte count associated with an erythroblastic hyperplasia; these characteristics indicate a completely ineffective erythropoiesis. The results of the study of haemoglobin synthesis performed on the bone marrow in vitro showed a greater imbalance in chain synthesis than that typically found in Cooley's anaemia. Ultrastructural studies revealed, in erythroblasts, all the features observed in Cooley's anaemia, although these features were more widely encountered. In addition, two peculiar findings were noted: (1) many inclusion bodies, which were partially or totally surrounded by smooth membranes, and which may indicate an autophagic phenomenon; and (2) a spongy appearance of the chromatin in rare erythroblasts of one of the two patients which resembled that found in congenital dyserythropoietic anaemia type I. These findings suggest that the great imbalance in the synthesis of the haemoglobin chain is responsible for the presence of an increased number of inclusion bodies and results in the death of nearly all the late erythroblasts. Furthermore, the present results point out the need for further study of the synthesis of the globin chains in atypical congenital dyserythropoietic anaemias.