We report on 19 patients with Waldenström's macroglobulinemia (WM) who were studied cytogenetically at the onset and during progression of the disease. We found a high frequency of chromosome changes confirming the claim of other authors that, during progression of the disease, a large number of residual neoplastic cells, insensitive to conventional chemotherapy, persist. In turn, this may be the cause of the difficulty of inducing remission (21% of cases) and of the short survival (mean, 35 months). In our experience it is difficult to identify the primary chromosome abnormalities because of the late clinical stage at which the chromosomes were examined. However, changes involving chromosomes #10, #11, and #12 may be unfavorable events in patients with WM.