By circumventing the need for metaphase preparations, fluorescent in situ hybridization (FISH) on interphase nuclei using chromosome-specific probes is a promising tool for the study of numerical chromosome aberrations not only in proliferating, but also in non-dividing cells. We analyzed 15 cases of monosomy-7-associated myeloid disorders with a biotinylated probe to the (peri)centromeric region of chromosome 7. Monosomy 7 was readily confirmed in all cases during active disease. In two patients only a minority of nuclei was monosomic, whereas cytogenetics had shown all metaphases to be missing one chromosome 7. FISH in one of them was able to identify a small marker chromosome as isolated pericentromeric region of chromosome 7. Minimal residual disease however could not be detected in three remission samples analyzed, as percentages of disomic nuclei were within the range of normal controls (96.8% 2.1%). In order to determine lineage involvement of the monosomic clone, a recent technique combining immunophenotyping and FISH (FICTION) was performed in one patient with AML after MPD. Monosomy 7 was found in virtually all myelomonocytic and erythroid cells (as discriminated by lineage-specific antibodies), in a part of CD34-positive precursor cells, but not in lymphocytes. We conclude that monosomy 7 in this patient is restricted to an early committed progenitor cell capable of erythroid and myelomonocytic differentiation.