The presence of a zone 4 rearrangement in the bcr of two chronic myelogenous leukemia (CML) cell lines and 25 CML patients was investigated by hybridization of HindIII-EcoRI double-digested DNA with the homologous HindIII-BglII breakpoint cluster region (bcr) probe. A rearrangement in zone 4 was found in the EM2 cell line and in 11 of the 25 patients. The presence of a zone 4 rearrangement was not associated with phase of disease, clinical evidence of acceleration, or pathologic evidence of acceleration. The difference between the actuarial duration of chronic phase for patients without (33 months) and with (22 months) a zone 4 rearrangement was not significant. A breakpoint in zone 4 of the bcr in patients with CML does not appear to be a determinant of prognosis.