Bone marrow and blood from three patients with myelodysplastic syndrome (MDS) and monosomy 7 were studied for cell lineage involvement of the chromosomal abnormality. Cytogenetic involvement of the myeloid and erythroid cell lineages in MDS with monosomy 7 has been shown before. Lymphoid subpopulations have also been investigated but generally with negative results. A combined technique of May-Grünwald-Giemsa (MGG) for cell cytology and interphase fluorescence in situ hybridization (FISH) using a chromosome 7 specific DNA probe was applied. Further, immunophenotype and genotype of the cells were simultaneously examined with alkaline phosphatase anti-alkaline phosphatase (APAAP) immunostaining and FISH. The monosomy 7 was found in the blasts and in all or in subpopulations of myeloid and erythroid cells. T cells (CD3+, CD5+) did not appear to be involved. B cells (CD19+, CD22+) showed a normal distribution of FISH spots in two patients. In one patient however the loss of a chromosome 7 was found in approximately 70% of the cells positive for B cell markers including CD79a. The results of this study show that in some cases MDS is a disease arising in a progenitor cell with repopulative abilities restricted to myelopoiesis and erythropoiesis. In other cases, the pluripotent progenitor cells in MDS may show the capacities to differentiate into B lineage lymphoid cells, as well as suggesting that in those instances MDS represents a condition of more primitive transformed hematopoietic ancestor cells.