Mediastinal lymphoma arises from the thymus gland or lymph nodes in the mediastinum — the central compartment of the chest between the lungs. It is strongly associated with feline leukaemia virus (FeLV) infection, particularly in young cats. The tumour typically consists of malignant T-lymphocytes and grows as a large mass in the chest cavity, often causing fluid to accumulate around the lungs (pleural effusion). Affected cats present with difficulty breathing, sometimes quite suddenly and severely. Siamese cats can develop a FeLV-negative form. Chemotherapy can produce meaningful remission in some cases, and FeLV-negative cats generally have a better response than FeLV-positive cats. The condition is primarily seen in younger cats (typically between 2 and 5 years of age), in contrast to most other feline lymphoma types which tend to affect older cats.