Castleman's disease is a morphologically and clinically heterogeneous lymphoproliferative disorder. Both a localized benign variant and an aggressive form with systemic manifestations have been described. To investigate the differences between these variants of Castleman's disease, the authors analyzed lymph node DNA from 4 patients with the localized type and 4 with the systemic type of Castleman's disease for immunoglobulin and T-cell receptor gene rearrangements. The role of Epstein-Barr virus (EBV) and cytomegalovirus (CMV) was also studied by viral genomic DNA probes. They detected clonal rearrangements in 3 of the 4 patients with the systemic variant of Castleman's; no patients with localized disease had rearrangements. Copies of EBV genome were also detected in 2 of the 3 patients with clonal rearrangements. These results suggest that systemic Castleman's disease is a disorder distinct from the classical localized variant in that it may evolve into a clonal lymphoproliferation.