Although numerous treatment approaches are proposed for patients with follicular lymphoma, criteria to help in choosing a treatment for a given patient and for comparing trial results are lacking. Several retrospective studies have analyzed prognostic factors, but their conclusions rely on limited numbers of patients treated during long periods, and their results are discordant. The Follicular Lymphoma International Prognostic Index was designed from the data recorded over 8 years of nearly 5000 patients registered worldwide. Five factors are used (age, Ann Arbor stage, number of nodal sites, serum lactate dehydrogenase level, and hemoglobin level) to build a three-category index. This index, together with new biologic markers such as gene profiling and proteomics, could help provide an optimal treatment option for patients with follicular lymphoma.