Myelodysplastic syndrome (MDS) is a well-recognized complication of chemotherapy for multiple myeloma (MM). Serial bone marrow metaphase examinations were performed for MM restaging in 3,077 patients undergoing high-dose therapy (HDT). MDS-associated cytogenetic abnormalities (MDS-CAs) were observed in 105 of 2,418 patients in whom cytogenetic data were available after HDT. MDS-CAs occurred transiently in 72 patients and on 3 successive occasions (persistent MDS-CAs) in 33 patients, for 10-year estimates of 4% and 2%, respectively; only 21 patients developed overt clinical MDS and 5, acute myeloblastic leukemia (AML). MDS-CA development was linked to lower CD34 yield at collection, longer time interval from MM diagnosis to HDT, older age, and lower platelet recovery after HDT; persistent MDS-CAs were predicted by CD34 yield of less than 3 x 10(6)/kg and need for more than 2 apheresis procedures. Applying a tertile frequency distribution over time to all 105 patients with MDS-CAs, its detection early after HDT was associated with longer time interval from diagnosis and low pre-HDT platelet count (likely resulting from pre-HDT damage), whereas late-onset MDS-CAs were noted among patients treated with Total Therapy 2 and Total Therapy 3 that applied post-HDT consolidation chemotherapy (suggesting possible post-HDT damage). While the risk of MDS-CAs was low and clinical MDS occurred infrequently, monitoring after post-HDT consolidation chemotherapy appears warranted.