Six cases of Waldenstrom''s macroglobulinemia are presently under investigation. All of them show, in a percentage of their blood cells, an extra large submetacentric chromosome. Preparations from 2 of these cases were analyzed by G[Giemsa]-banding techniques, and about 2% of the metaphases in these preparations show 47 chromosomes; the extra chromosome appears to be a 3 with a deletion on the distal part of the short arm.