In very rare patients tetrasomy of chromosome 8 is observed as the only karyotypic event. We present a 74-year-old man with acute monocytic leukemia (FAB M5) displaying 46,XY/48,XY, +8, +8 in unstimulated bone marrow cells. The patient died 2 weeks after diagnosis. Tetrasomy 8 appears to identify a subgroup of acute nonlymphocytic leukemia. It may indicate a poor prognosis.