Up to now only limited information is available about the significance of chromosomal aberrations in multiple myeloma (MM) and about the time point of the neoplastic transformation. Fluorescence in situ hybridization (FISH) combined with standard May-Grünwald-Giemsa (MGG) staining or immunophenotyping on a single cell level were applied. Bone marrow (BM) samples were obtained from 11 patients with morphologically proven multiple myeloma. For detection of the chromosomal aberrations, we used FISH on interphase nuclei with commercially available centromere-specific probes for chromosomes 1, 7, 9, 11, 15, and 17 and further DNA probes for 5p13, 5q31, Rb-gene (13q14), cyclin D1 gene (11q13), and p53 gene (17p13). The aberration rate differed between 14% and 71% on bone marrow smears. Using the combination of MGG and FISH we analyzed eight patients. A total of 2622 bone marrow cells were morphologically identified and investigated for their specific chromosomal aberrations. For all probes applied, 57 cells of the erythropoietic lineage, 698 cells of the granulopoietic lineage, and 168 lymphocytes showed two normal FISH signals. Of 1723 nuclei of plasma cells, 464 (26.9%) were also not aberrant, whereas all other nuclei of plasma cells (n=1259, 73%) showed a specific aberration. Combination of fluorescence immunophenotyping and in situ hybridization (FICTION) was applied in 10 of 11 patients. Seventy-eight investigated CD34-positive precursor cells did not show any specific aberration detected before in the plasma cell compartment. In conclusion, the combination of MGG and FISH on a single cell level demonstrated that only plasma cells bore the chromosomal aberration and MM did not evolve at an early cell level.