We report on eight patients who were 35 to 77 years old with an isochromosome 17q as the sole structural chromosomal anomaly. Additional numerical chromosomal changes were a trisomy 8 or 17 in two cases each and a trisomy 19 in one case. Five patients had myelodysplastic syndrome (MDS) diagnosed according to the FAB nomenclature as chronic myelomonocytic leukemia (CMML) in two cases, refractory anemia with excess of blasts in transformation (RAEBt) in two cases, and refractory anemia with excess of blasts (RAEB) in one case. One patient suffered from a myeloproliferative disorder (MPS). All cases progressed to acute nonlymphocytic leukemia (ANLL) type M1, M2, or M4 in a period of 2 to 30 months after initial diagnosis, except one patient with RAEBt who died within 2 months. Two patients presented with ANLL-M2 at time of diagnosis. Treatment during the chronic phase of disease consisted of mild cytoreduction and/or substitution of platelets or red blood cells. One patient with CMML received an allogeneic bone marrow graft and relapsed after 33 months with ANLL-M1. Treatment results for overt leukemia were poor, and survival was short, lasting from 1 to 4 months. Overall survival was 1 to 37 months (median duration, 6.5 months). Molecular studies in two cases revealed neither a BCR rearrangement nor a translocation of the ABL protooncogene, as observed in Ph1-positive chronic myeloid leukemia (CML). Thus, an i(17q) anomaly seems to identify a distinct subgroup of mostly myelodysplastic and, less frequently, myeloproliferative disorders that progress rapidly to ANLL, respond poorly to chemotherapy, and are associated with short survival after transformation.