To investigate the genetic basis of the great heterogeneity observed in the clinical behavior of multiple myeloma (MM), a combined approach of G‐banding, interphase fluorescence in situ hybridization (FISH), and multicolor FISH (M‐FISH) was employed to analyze 70 samples from 53 patients with MM. G‐banding revealed abnormal karyotypes in 77% of the cases. The origin of 31 chromosome markers was identified or revised by M‐FISH. Combined metaphase karyotypic data and interphase FISH findings, using the immunoglobulin heavy‐chain (IGH), IGH/cyclin D1 gene (CCND1), and D13S319 probes, revealed chromosome abnormalities in all evaluated patients and marked inter‐ and intratumor cytogenetic heterogeneity in the investigated MM samples. Cytogenetically unrelated clones were detected in 26% of the cases, mostly MM evaluated at diagnosis, whereas cytogenetic clonal evolution, manifested as related clones in 20% of the cases, was associated with disease progression. Among the 14q32 rearrangements, present in 66% of the cases, at least three cytogenetic subsets could be identified: one with t(11;14), usually without 13q14 deletion; another with other IGH changes, often 13q14 deletion, and hypodiploid modal chromosome number; and a third without changes in 14q32 but with abnormalities of chromosome 17. The correlation found between cytogenetic and clinicopathologic characteristics provided support for the concept that general genomic features in conjunction with specific chromosome rearrangements define the malignant phenotype in the various subsets of MM. © 2004 Wiley‐Liss, Inc.