In malignancies with a low mitotic index such as multiple myeloma (MM), conventional cytogenetic studies may not be informative. This study's purpose was to assess specific numerical chromosomal aberrations in non-dividing MM cells by fluorescence in situ hybridization (FISH) of DNA chromosome probes on bone marrow smears. Old air-dried bone marrow smears from 18 MM patients were probed with alpha satellite DNA sequences for chromosomes 7, X, and Y, and a whole painting probe for chromosome 11. Plasma cells were identified by their morphologic characteristics so that counts of fluorescent signals in the nuclei of MM cells could be differentiated from those of normal marrow cells. Numerical chromosome aberrations were found in 66.7% of the cases (12 of 18), including 5 cases of trisomy 7, 2 cases of tetraploidy, 2 cases of monosomy X in females, 2 cases of disomy X in males, and 1 case of nullisomy Y. In addition, 2 of the 7 cases probed with chromosome 11 paint demonstrated 3 signals in about 15% of the cells. This study illustrates the advantages of FISH for interphase analysis of chromosome aberrations in slowly dividing cells, as well as the ability to use old slides for retrospective studies.