Lymphoma is a group of blood cell tumors that develop from lymphocytes. It is sometimes used to refer to just the cancerous ones rather than all tumors. Symptoms may include: enlarged lymph nodes that are not generally painful, fevers, sweats, itchiness, weight loss, and feeling tired, among others. The sweats are most common at night.

The two main types are Hodgkin lymphoma (HL) and non-Hodgkin lymphoma (NHL), with two others, multiple myeloma and immunoproliferative diseases, also included by the World Health Organization (WHO) within the category. Non-Hodgkin lymphoma makes up about 90% of cases and includes a large number of subtypes. Lymphomas are part of the broader group of neoplasms called tumors of the hematopoietic and lymphoid tissues.

Risk factors for HL include: infection with Epstein–Barr virus and having others in the family with the disease. Risk factors for NHL include: autoimmune diseases, HIV/AIDS, infection with human T-lymphotropic virus, eating a large amount of meat and fat, immunosuppressant medications, and some pesticides. They are usually diagnosed by blood, urine, or bone marrow testing. A biopsy of a lymph node may also be useful. Medical imaging then may be done to determine if and where the cancer has spread. This spread can occur to many other organs, including: lungs, liver, and brain.

Treatment may involve some combination of chemotherapy, radiation therapy, targeted therapy, and surgery. In NHL, the blood may become so thick with protein that a procedure called plasmapheresis is needed. Watchful waiting may be appropriate for certain types. Some types are curable. The overall five-year survival rate in the United States for HL is 85%, while that for NHL is 69%. Worldwide, lymphomas developed in 566,000 people in 2012 and caused 305,000 deaths. They make up 3–4% of all cancers, making them as a group the seventh-most common form. In children they are the third most common cancer. They occur more often in the developed world than the developing world.