Fluorescent in situ hybridization (FISH) is a technique which complements conventional cytogenetic banding analysis by allowing the evaluation of cells in interphase as well as metaphase. This technique has been used to study air-dried peripheral blood and bone marrow aspirate smears. We have applied the FISH technique to study routinely processed sections of bone marrow aspirate clot and decalcified core biopsy specimens, fixed in either formalin or B5 and embedded in paraffin. We evaluated 28 specimens (8 aspirate clot and 20 core biopsy sections) for chromosome 8 copy number, studied previously by conventional cytogenetics, and found the following distribution: 15 with disomy, 11 with trisomy, and 2 with tetrasomy. Using a chromosome 8 alpha-satellite probe, we detected fluorescent hybridization signals in 18 of 28 specimens (64%); 6 of 8 (75%) aspirate clot sections, and 12 of 20 (60%) core biopsy sections. Ten of 13 (77%) B5-fixed and 8 of 15 (53%) formalin-fixed specimens had hybridizing signals. Specimen age was a significant factor; 10 of 11 (91%) specimens processed within the last 6 months showed signals, in contrast with 8 of 17 (47%) specimens older than 6 months. In the positive specimens, 200 cells were analyzed in areas where individual cells could be identified. In the disomic specimens, two signals per cell were seen in 34 to 66% of the cells. Rare cells (0-2%) with three signals were detected. In the trisomic specimens, three signals per cell were seen in 19 to 46% of the cells. In the tetrasomic specimens, four signals per cell were seen in 15 to 25% of the cells. We conclude that the FISH technique may be useful in the detection of numerical chromosomal abnormalities such as trisomy and tetrasomy 8 in routinely processed bone marrow aspirate clot and decalcified core biopsy sections.