In many B-cell malignancies, 14q32.3 chromosomal rearrangements involving the immunoglobulin heavy chain (IgH) locus have been shown to be pathognomonic for the disease. Although in myeloma heterogeneous and complex karyotypes are found, 14q32.3 translocations are prominent. However, owing to the telomeric position of the IgH locus, 14q32.3 translocations may be easily missed. We established fluorescence in situ hybridization (FISH) assays on chromosomes and DNA fibers to determine both the occurrence of 14q32.3 rearrangements in myeloma cell lines and the precise localization of the breakpoints in the IgH locus. Our results show that 14q32.3 chromosomal rearrangements are present in almost every myeloma cell line analyzed (17 of 19, 89%). Breakpoint analysis of the lines harboring one or more 14q32.3 rearrangements with the use of fiber-FISH revealed the involvement of switch regions in the IgH locus in 11 of 17 cell lines. Remarkably, pseudogamma genes without switch regions were involved in 3 of 17 cell lines, all derived from IgA myelomas. Three of 17 cell lines contained breakpoints outside a switch or immunoglobulin heavy chain constant region. The almost ubiquitous presence of 14q32.3 rearrangements suggests an obligatory role in the development of myeloma. The high incidence of breakpoints involving switch regions indicates an oncogenic event in a late stage of B-cell differentiation.