Trisomy 7 has been found as the sole chromosomal anomaly in both benign and malignant tumors, as well as in nonneoplastic lesions. It has been reported that +7 may occur in tumor-infiltrating as well as in peripheral T cell and in the thymus. The precursor cells, which mature in the thymus to T lymphocytes, originate in the bone marrow and the present study was undertaken to investigate whether cells carrying an extra chromosome 7 can be detected there. Bone marrow samples from five hematologically normal individuals and 12 children with acute lymphoblastic leukemia (ALL) were analyzed by interphase fluorescence in situ hybridization (FISH) using a chromosome 7 centromere-specific probe. Half of the ALLs were karyotypically normal, whereas the other half displayed clonal abnormalities (one pseudodiploid and five hyperdiploid karyotypes, none of which had +7). The FISH analysis showed no evidence of cells with trisomy 7 in the bone marrow samples from the controls or ALLs. This suggests that the gain of a chromosome 7 by T lymphocytes does not occur before the bone marrow precursor cells are conditioned in the thymus.