Classification and differential diagnosis of erythroid neoplasias still are a matter of discussion. Eleven cases of primary acute erythremia were diagnosed between 1981 and 1984 at the Institute of Pathology, University of Kiel. Erythremia represented 0.5% of all hematological diagnoses and 1.0% of the myeloproliferative disorders. The male-to-female ratio was 1:1. Incidence peaked in the 7th decade. Evaluation of clinical data, of cytological and histological findings in blood and bone marrow, and of occasional immunophenotyping of blast cells (anti-glycophorin A+) revealed two variants of acute erythremia: a first, blastic one and a second, more differentiated form. Acute erythremia must be strictly distinguished from mixed erythroid/myeloid erythroleukemia and from secondary erythroid neoplasias, especially the erythremic 'blast crisis' of chronic myeloid leukemia or polycythemia vera rubra. Distinguishing the myelodysplastic variant of sideroblastic anemia from anerythremic acute erythremia can be extremely difficult. We discuss the differential diagnosis and classification of erythroid neoplasias based upon reproducible hematological criteria to facilitate the gathering and comparison of epidemiological and clinical data on these rare malignancies.