A cancer of the lymphocytes and the lymphatic system. There are two main types: Hodgkin's and Non-Hodgkin's. Non-Hodgkin's is 5 times more common. The disease occurs when the lymphocytes of the immune system grow out of control becoming cancerous. The overgrowth of the lymphocytes typically causes swelling of lymph nodes. The cancerous lymphocytes can spread to other parts of the body (metastasize).