Twenty-three samples of benign and malignant bone tumors were studied with cytogenetic analysis, interphase cytogenetics (IC) using in situ hybridization with (peri)centromeric probes for chromosomes 1, 7, and/or 8, and DNA flow cytometry (FCM). Our aim was to compare these methods in the detection of numerical chromosome aberrations (NCA) and aneuploidy. IC detected aneuploidy in 91%, FCM in 73%, and cytogenetics in 27% of the malignant tumors. In benign tumors IC detected aneuploidy in 4 (33%), FCM in 2 (17%), and cytogenetic analysis in 1. All of the benign tumors aneuploid by IC, two of which were also aneuploid by FCM, were histologically potentially aggressive. The clonal aberrations detected with cytogenetics usually agreed with the IC and FCM findings. All malignant tumors which had a normal karyotype were aneuploid either by IC or FCM or by both. In conclusion, IC was the most sensitive method in the detection of NCA and aneuploidy even though it was usually performed with only two (peri)centromeric probes. Aneuploidy was detected by cytogenetic analysis alone in 4 samples (1 7%), by cytogenetic analysis and/or FCM in 1 1 samples (a%), and by cytogenetic analysis, FCM, and/or IC in 16 samples (70%). Thus, the combined use of all three methods increased the sensitivity of aneuploidy detection.