Transformed lymphoma has a reported incidence of 10-70% among patients with follicular lymphoma. Interpreting the wide-ranging estimates for incidence, survival, and effects of interventions from various studies is complicated by the use of different definitions of lymphoma transformation. Problems in defining histologic transformation will be addressed in this review. To date, there are no reliable markers of risk for transformation or factors predictive of survival. The prognosis for transformed lymphoma is generally poor, with most patients surviving only a few months, though some with limited disease that is chemosensitive may experience prolonged survival. Immunotherapy, particularly monoclonal antibodies and radioimmunoconjugates, holds promise but more experience is necessary. Approximately 200 patients are included in published series of autologous transplantation; of these, one-third remain disease free at 5 years, not dissimilar to reported outcomes for nontransformed disease. However, the treatment-related mortality is higher than in nontransformed disease, and there is a significant incidence of post-transplant myelodysplastic syndrome. The role of allogeneic transplant has yet to be pursued, but should be explored for its potential for a graft-versus-lymphoma effect. Advances in microarray gene analysis and biology may facilitate the understanding of mechanisms of transformation, development of a prognostic index and creation of tailored therapy.