A patient with acute monoblastic leukemia (AML M5a) and the pericentric inversion inv(8)(p11q13) as well as additional chromosome abnormalities in her bone marrow cells is described. This is the fourth known case of inv(8)(p11q13)-positive acute leukemia, and the second such case in which gain of 1q material occurred during clonal evolution. All patients with acute leukemia and inv(8)(p11q13) have been females, most have been young, and there has been a tendency for the disease to run an aggressive course. Both hematologically and cytogenetically, therefore, inv(8)(p11q13)-positive leukemia may be viewed as a variant of AML with t(8;16)(p11;p13). This similarity is also apparent at the molecular genetic level, in-as-much as the MOZ gene in 8p11 is rearranged in both the translocation and the inversion; in t(8;16)-positive leukemia, a MOZ-CBP chimeric gene is generated, whereas inv(8) has been shown to generate a MOZ-TIF2 fusion gene. Southern blot analysis of the present case after MOZ0.8 hybridization of Bam HI digested DNA gave an 11 kb aberrant band in addition to the germline band, corresponding to a breakpoint immediately upstream of the 4 kb long MOZ exon that begins at position 3746. Also previously investigated inv(8)-positive leukemias have shown breaks in this intron indicating that it contains sequence motifs predisposing to illegitimate recombination.