A patient with severe anemia, reticulocytopenia, and erythroid hyperplasia of the bone marrow developed fatal acute nonlymphocytic leukemia after 3 yr. A Philadelphia chromosome with the typical 9/22 translocation t(9q +;22q-) was identified by banding techniques in a small number of bone marrow cells throughout the preleukemic phase of the illness (14%--38% of metaphases) and during the acute transformation (50%). Granulocytic colony formation in vitro was abnormal in the preleukemic phase. The diagnosis of chronic granulocytic leukemia was excluded on the basis of clinical and laboratory findings. The identification of the Ph1 chromosome in this form of chronic myelodysplastic syndrome (preleukemia) provides a new example of a hematologic disorder predisposing to acute leukemia in which this chromosomal abnormality occurs.

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