Immunoglobulin and T-cell receptor gene rearrangement assays are sensitive methods of detecting clonality in lymphoproliferative disorders. The lack of lineage specificity of immunoglobulin heavy chain and T-cell receptor beta and gamma gene probes in acute leukemia is well established. However, immunoglobulin light chain gene rearrangement traditionally has been considered a highly specific indicator of a clonal B-lineage process. The authors describe a case of T-cell acute lymphoblastic leukemia in which Southern blot hybridization studies showed rearrangement of the T-cell receptor beta chain gene. Unexpectedly, the immunoglobulin kappa light chain gene also was rearranged; no immunoglobulin heavy chain gene rearrangement was seen. Kappa rearrangement was confirmed with the use of three restriction endonucleases. No rearrangements were seen from normal skin tissue, making a restriction enzyme site polymorphism highly unlikely. Northern blot studies showed a normal-size, T-cell receptor beta chain gene transcript; no immunoglobulin RNA was identified. These results describe the first reported case of kappa light chain gene rearrangement in a T-cell acute lymphoblastic leukemia. The findings emphasize the necessity of interpreting molecular hybridization studies in conjunction with routine morphology and immunophenotyping studies.