The authors report a case of complete lymph node infarction in which a specific etiology could not be determined by morphologic or immunophenotypic studies; however, clonal rearrangement of the immunoglobulin gene was demonstrated by Southern blot hybridization of DNA extracted from the necrotic tissue. A subsequent lymph node biopsy later was diagnosed as malignant lymphoma, using morphologic, immunophenotypic and genotypic criteria. Identical clonally rearranged bands were present in DNA from both the infarcted nodal and the subsequent tissue biopsies. In the setting of lymph node necrosis, gene rearrangement studies may provide diagnostic information concerning clonality, even if morphologic and immunophenotypic studies are indeterminate for a lymphoproliferative process.