A putative isochromosome of the long arm of chromosome #17 was identified in bone marrow cells from each of six patients with leukemia. Adequate sample was available for detailed study in four patients, and in each of these four cases examination with multiple staining techniques implied that the rearrangement is a dicentric chromosome, dic(17)(p11.2). An asymmetry of constriction at the two centromeric regions suggested that one centromere was inactive. The data presented here suggest that many of the presumed i(17q) markers observed in leukemia may actually represent dicentric rearrangements.