Follicular lymphoma (FL) is a cancer that involves certain types of white blood cells known as lymphocytes. The cancer originates from the uncontrolled division of specific types of B-cells known as centrocytes and centroblasts. These cells normally occupy the follicles (nodular swirls of various types of lymphocytes) in the germinal centers of lymphoid tissues such as lymph nodes. The cancerous cells in FL typically form follicular or follicle-like structures (see adjacent Figure) in the tissues they invade. These structures are usually the dominant histological feature of this cancer.

There are several synonymous and obsolete terms for FL such as CB/CC lymphoma (centroblastic and centrocytic lymphoma), nodular lymphoma, Brill-Symmers Disease, and the subtype designation, follicular large-cell lymphoma. In the US and Europe, this disease is the second most common form of non-Hodgkin's lymphomas, exceeded only by diffuse large B-cell lymphoma. FL accounts for 10–20% of non-Hodgkin's lymphomas with ~15,000 new cases of it being newly diagnosed each year in the US and Europe. Recent studies indicate that FL is similarly prevalent in Japan.

FL is a broad and extremely complex clinical entity with a wide range of manifestations which have not yet been fully systematized. It is commonly preceded by a benign precancerous disorder in which abnormal centrocytes and/or centroblasts accumulate in lymphoid tissue. They may then circulate in the blood to cause an asymptomatic condition termed in situ lymphoid neoplasia of the follicular lymphoma type (i.e. ISFL). A small percentage of these cases progress to FL. Most commonly, however, FL presents as a swelling of lymph nodes in the neck, armpits, and/or groin. Less often, it presents as a gastrointestinal tract cancer, a cancer in children involving lymphoid tissues of the head and neck area (e.g. tonsils), or one or more masses in non-lymphoid tissues such as the testes.

FL typically has a slow disease course which persists essentially unchanged for years. However, each year 2–3% of FL cases progress to a highly aggressive form often termed stage 3B FL, to an aggressive diffuse large B-cell lymphoma, or to another type of aggressive B-cell cancer. These transformed follicular lymphomas (t-FL) are essentially incurable. However, recent advancements in the treatment of t-FL (e.g. the addition to standard chemotherapy of agents such as rituximab) have improved overall survival times. These newer regimens may also delay the transformation of FL to t-FL. Additional advances in understanding FL may lead to further improvements in treating the disease.