Cytogenetic and immunologic studies were performed on the cells of an 18-year-old female with ataxia telangiectasia (AT) associated with acute lymphocytic leukemia (ALL). At the onset of the leukemia 15.4% of peripheral blood cells stimulated with phytohemagglutinin (PHA) contained a tandem translocation of the long arm of chromosome #14, i.e., t(14;14). To ascertain if these karyotypically abnormal cells and the leukemic cells had a common lineage, chromosome analyses were performed on bone marrow cells. Examination of the marrow cells on the seven occasions when leukemic cells were present in the marrow, including times when they were predominant, showed only a normal karyotype without the presence of t(14;14). However, an abnormal clone, which had the karyotype 45,XX,-9,t(9;6)(q12;q13), was identified in the marrow cells on the last examination during the terminal phase of the leukemia. Immunologically, the ALL was classified as an atypical type which had characteristics in common with certain T-cell subsets. We suggest that the malignant cells did not originate from the preexisting cells with a tandem duplication of the 14q.