Using fluorescence in situ hybridization (FISH), we studied chromosome 8 abnormalities in 30 children with mature B-cell acute lymphoblastic leukemia (B-ALL) or B-cell non-Hodgkin lymphoma (B-NHL). FISH was performed on metaphase spreads and interphase nuclei with a whole chromosome 8 painting probe. Fifteen patients were studied retrospectively after metaphases from the malignant cell specimen had already been G-banded. When interphase nuclei were examined, FISH was able to confirm t(8;14)(q24;q32) in all nine patients positive by previous G-banding. FISH, however, was positive in metaphase spreads from only seven patients. Another 15 patients were included in a prospective study. In six of them (40%), a translocation involving chromosome 8 was shown by a split small segment (8q24-8qter) on interphase nuclei. Analysis of metaphase spreads showed only three positive cases each by FISH or G-banding, respectively, with corresponding results in two patients. By interphase FISH, trisomy of chromosome 8 also was detectable. In three patients shown by G-banding to have trisomy, interphase FISH study showed high scores of three chromosome 8 signal positive cells. There was no cross-hybridization to other chromosomes interfering with FISH analysis. FISH analysis on interphase nuclei using a whole chromosome 8 hybridization probe will supplement and can be more sensitive than metaphase cytogenetic techniques for detection of chromosome 8 rearrangements in B-ALL/NHL.