Lymphomas are a group of blood cell tumors that develop from lymphatic tissues. The name often refers to just the cancerous ones rather than all such tumors. Symptoms may include enlarged lymph nodes that are not generally painful, fevers, sweats, itchiness, weight loss, and feeling tired. The sweats are most common at night.

There are dozens of subtypes of lymphomas. The two main categories of lymphomas are Hodgkin lymphomas (HL) and the non-Hodgkin lymphomas (NHL). The World Health Organization (WHO) also includes two other categories, multiple myeloma and immunoproliferative diseases, as types of lymphoma. The many subtypes of non-Hodgkin lymphomas make up about 90% of all lymphoma cases. Lymphomas and leukemias are part of the broader group of tumors called tumors of the hematopoietic and lymphoid tissues

Risk factors for the Hodgkin lymphomas include infection with Epstein–Barr virus and having others in the family with the disease. Risk factors for common types of non-Hodgkin lymphomas 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 some of the non-Hodgkin lymphomas, 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 outcome depends on the subtype. The overall five-year survival rate in the United States for the Hodgkin lymphomas is 85%, while that for non-Hodgkin lymphomas averages 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.