We followed a 2-year-old girl with erythroleukemia (EL) for 7 months, from the time of her initial diagnosis until her death. Immunophenotyping of bone marrow was negative for markers of myeloid and lymphoid lineages. Chromosome study of marrow at diagnosis revealed abnormalities in all mitotic cells, with a clonal karyotype of 48,XX,t(2;12)(p11.2; p13),+6,+21. Subsequent studies showed that the clone rapidly evolved and accumulated additional structural and numerical abnormalities, in spite of intensive chemotherapy during the final months. Simultaneous study of the karyotype and cell morphology of dividing bone marrow cells after 24 hours of in vitro culture, using a technique that preserves cell structure, showed that cells with the chromosome abnormalities were PAS-negative. The combined immunological, cytogenetic, and morphological data indicate that the original malignant cell type was neither myeloblast nor erythroblast but an early progenitor type that rapidly proliferated.