- Introduction Congenital dyserythropoietic anemias are rare blood disorders characterized by congenital anemia and a wide range of morphological and functional abnormalities of erythroid precursors. Objectives To analyze the relative frequency of both light microscopic (LM) and electron microscopic (EM) morphological features of erythroblasts in a large group of patients with molecular proven congenital dyserythropoietic anemia type I (CDAI). Methods We retrospectively evaluated the LM and EM of bone marrow (BM) erythroblasts in 35 patients with CDAI. Thirty-four patients carried the CDAN1 Arg1042Trp founder mutation and one the p.Pro1130Leu mutation. BM slides of 24 patients were available for LM examination. EM studies were performed in all 35 patients. Results On LM, marked erythroid hyperplasia, binuclear erythroblasts and various non-specific dyserythropoietic features were documented in every case; internuclear chromatin bridges were detected in 19 patients (79%). In all EM of erythroblasts revealed a spongy appearance of heterochromatin, a widening of nuclear pores and invagination of cytoplasm into the nuclear region. Conclusions EM studies revealed high morphological frequency of specific ultrastructural changes in erythroblasts which facilitate prompt diagnosis of CDAI. Due to low specificity of BM LM findings, when BM EM is unavailable diagnostic approach should also include other inherited anemias. This article is protected by copyright. All rights reserved.