The case of a 4-year-old boy with ALL, possibly developing subsequent to a lymphoma involving the thoracic cavity, which was shown to be of the T-cell type, is presented. The leukemic cells had a 5q-- anomaly, which had previously been described only in cases of refractory anemia and/or AML. The 5q-- abnormality was invariably accompanied by a 9p-- chromosome in the leukemic cells. The interstitial deletion leading to the 5q-- chromosome was shown with banding techniques to be similar to that described previously in myeloproliferative disorders. Some aspects of the 5q-- anomaly in ALL and its relation to previous experience with this karyotypic change are discussed.