Chromosome 13 abnormalities are frequently observed in multiple myeloma (MM). Several reports recently demonstrated the strong prognostic value of these abnormalities, associated with a short survival. Cytogenetic studies have shown that most of these abnormalities are complete monosomies. In order to define the common minimal deletion, we analysed a series of 234 patients with MM using fluorescence in situ hybridization (FISH) with a panel of five probes mapping along the whole chromosome 13. A chromosome 13 abnormality was observed in 98 patients (42%), 90 of whom (92%) displayed a complete monosomy. In seven of the eight remaining patients presenting partial deletions, the three probes specific for the 13q14 region were deleted. Only one patient (1%) displayed a small deletion of the D13S319 locus. In conclusion, FISH should be used for the analysis of chromosome 13 abnormalities, using probes mapping in the 13q14 region.