Leukaemia can present with joint swelling in the absence of abnormal haematological findings. Arthritis as a presenting sign of lymphoma, however, is extremely rare. Three children with non-Hodgkin's lymphoma who had joint swelling at the onset of their disease are reported. Two cases showed histological features of anaplastic large cell lymphoma (Ki-l/CD30 positive), and one of angioimmunoblastic T cell lymphoma. In all patients the unusual presentation delayed correct diagnosis.