Recently a high incidence of interstitial deletion of the P53 locus has been reported in multiple myeloma (MM) patients. Considering the importance of such an event, we analysed 79 patients with advanced-stage disease using fluorescence in situ hybridization (FISH). Strikingly, we found only 7/79 patients with a P53 deletion. In order to rule out any differences in probe selection, we reanalysed all the patients with the same probe as that used in a previous study, and confirmed the low incidence of P53 deletion (7/79, 9%). The only explanation is a difference in hybridization efficiency. Since hybridization is far less efficient on malignant plasma cells than on other bone marrow cells we suggest that this poor hybridization efficiency may lead to a false P53 deletion.