Monosomy for all or part of chromosome 7 in bone marrow mitoses of some patients with myelodysplastic syndrome or acute nonlymphocytic leukemia has been associated with a defect in granulocyte function. To study which blood-cell lineages are affected by the monosomy, we used chromosome 7-specific DNA probes in Southern blotting experiments on DNA derived from specific cell fractions isolated from the blood of five patients. As judged by the presence or absence of two different alleles for restriction-fragment-length polymorphisms, lymphocytes of all five patients were shown to have two different chromosomes 7. Granulocytes were affected by the chromosomal abnormality in four patients (No. 1, 2, 4, and 5) and unaffected in one (No. 3). Chemotaxis was normal in Patient 3 and impaired in Patients 4 and 5. Monocytes were affected by the monosomy in two of three patients (No. 2 and 3) and mainly unaffected in one (No. 1). Thus, the granulocytes and monocytes were affected differently in different patients. We conclude that mature blood cells are derived from abnormal progenitors and that there may be heterogeneity in the involvement of different cell lineages in different patients with myelodysplastic syndrome or acute nonlymphocytic leukemia. There is an association between DNA loss and functional impairment.