Multiple studies, with differing results, have compared the added sensitivity of fluorescence in situ hybridization (FISH) with conventional cytogenetics (CC) to detect genetic abnormalities in myelodysplastic syndrome (MDS). We hypothesized that in the setting of an adequate CC study, FISH would correlate with microscopic genetic abnormalities involving chromosomes 5, 7, 8, and 20. We performed FISH for -5/5q, -7/7q, +8, and del(20q) on 102 MDS cases with normal CC (> or =20 consecutive metaphases) and on 35 MDS cases with abnormal CC. Of the 102 MDS cases with normal CC, only 1 was discrepant between FISH (showing +8) and CC (<1% of total cases). Of the 35 MDS cases with abnormal CC, 1 showed a minor discrepancy (-5 by CC vs del(5q) by FISH). FISH for MDS abnormalities (-5/5q, -7/7q, +8, and del(20q)) correlates with an adequate karyotypic result without increased sensitivity. Consequently, we recommend that FISH not be performed in MDS cases with an adequate karyotype.