Chromosome change at 20q11-q12, including del(20q), is sometimes reported in plasma cell dyscrasia, but most cases are found during or after chemotherapy. It is therefore still uncertain whether del(20q) is a primary change or therapy-related. We performed cytogenetic studies and fluorescent in situ hybridization (FISH) analysis using 20q12 and 20qter probes to ascertain the possible involvement of 20q in nine patients with Waldenström macroglobulinemia (WM). The FISH study demonstrated deletions of 20q12 and/or 20qter in four of nine patients (44%) with WM at diagnosis, and one of them had the del(20q) chromosome. Moreover, one patient had de novo appearance of the del(20q) chromosome with 20q12 deletion after chemotherapy, although this patient had neither the del(20q) chromosome nor 20q12 deletion at WM diagnosis. Based on the results of this study, we conclude that chromosomal breakage at 20q13 is a non-random genetic change which plays a role in the neoplastic process of WM.