Fluorescence in situ hybridization (FISH) was performed on interphase bone marrow cells to study numerical chromosome abnormalities in childhood acute lymphoblastic leukemia (ALL). Ten patients were selected for this study on the basis of having an extra chromosome 6 in the abnormal clone of the bone marrow at diagnosis. The numerical changes that were detected by FISH with a chromosome 6 specific alpha-satellite DNA probe correlated well with the cytogenetic and clinical data in all patients. Three hybridization signals were seen in 43.8--83.0% of interphase cells in the specimens with a hyperdiploid karyotype. The diagnostic bone marrow sample of the patient with a tetraploid karyotype revealed four signals in 67.0% of cells. Two signals were detected in the majority of the cells in the three nonleukemic control bone marrow samples (97.0--97.7%), assessing the cut-off value of about 1% for trisomy 6. This study demonstrates that FISH analysis is a useful and sensitive tool to screen for the presence of extra chromosomes in interphase cells and is important clinically for evaluating the achievement and maintenance of remission in hyperdiploid childhood ALL. However, to detect structural chromosome aberrations which carry important diagnostic and prognostic information, as seen in our patient with the translocation t(1;19) at diagnosis but not at relapse, conventional cytogenetic analysis should be performed both at diagnosis and at relapse.