The International Non-Hodgkin's Lymphoma Prognostic Factor Index (INHLPF-Index) is a validated method of determining prognosis for patients with aggressive histology lymphoma and is now being used to identify those who might benefit most from new investigative therapies. This review assesses its usefulness and potential clinical utility in this context, and shows that several factors will determine the clinical utility of using this index, including the inherent accuracy of the index, the effect of mislabelling and the efficacy and toxicity of experimental treatment.