Evidence for chromosome #1 involvement in structural rearrangements in cancer is reviewed. There have been adequate studies of cancer at most of the common sites, and at all of these, nonrandom chromosome #1 involvement has been demonstrated. In general, a variety of changes is encountered, irrespective of the site; most commonly, however, the changes result in the duplication of long arm material. It seems that these nonrandom changes, which tend to occur at a relatively late stage, may contribute to the progression of all forms of cancer. However, a small number of chromosome #1 aberrations are also now known, which may represent specific and possibly initiating changes in particular forms of cancer. These include short arm deletions in neuroblastoma and translocations in leukemias and myelodysplasia.