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Trisomy 8 is a common anomaly in bone marrow (BM) cells of patients with myeloproliferative disorders (MPD), myelodysplastic syndromes (MDS), or acute nonlymphocytic leukemia (ANLL). We studied the efficacy of fluorescence in situ hybridization (FISH) detection of trisomy 8 in patients with MPD, MDS, or ANLL using directly labeled fluorescent alpha-satellite and whole chromosome paint (WCP) DNA probes specific for chromosome 8. Using FISH, we analyzed interphase nuclei and metaphase spreads from randomized series of BM specimens from normal individuals and patients with varying proportions of trisomy 8 as determined by conventional cytogenetic analysis. The BM of all normal donors contained less than or equal to 2.0% nuclei with 3 interphase FISH signals and less than or equal to 1 metaphase with 3 WCP FISH signals. Ninety-five percent and 98% of BM specimens with at least two metaphase cells with trisomy 8 by cytogenetic analysis contained greater than 2.0% nuclei with 3 interphase FISH and greater than 2 metaphases with 3 WCP FISH signals, respectively. Thirteen patients had 1 in 20 or 1 in 30 metaphase cells with trisomy 8 by conventional cytogenetic studies. Of these patients, four had greater than 2.0% nuclei with 3 interphase FISH signals. The BM of all four patients contained positive metaphase FISH results. We then studied the usefulness of FISH analysis to detect occult trisomy 8 by analyzing BM nuclei from 144 patients who had MPD, MDS, or ANLL and either 20 normal metaphase cells or an abnormal karyotype without trisomy 8. Seven patients had greater than 2.0% nuclei with 3 interphase FISH signals (range, 2.10% to 3.40%) and six patients had 2 or more cells with trisomy 8 upon metaphase FISH or extensive conventional cytogenetic analysis. Our results show that interphase and metaphase FISH analyses are useful methods to detect trisomy 8 cells in BM specimens, especially for specimens with normal or uncertain conventional cytogenetic results.