To investigate potential associations between several autoimmune diseases and haematological malignancies, we studied 39,908 cases of leukaemia, Hodgkin's disease, non-Hodgkin's lymphoma and myeloma that occurred during 1987-1999 in Sweden, and 149,344 controls. Hospital discharge diagnoses of psoriasis, Sjögren's syndrome, autoimmune haemolytic anaemia, idiopathic thrombocytopenic purpura, pernicious anaemia, multiple sclerosis, rheumatic fever or asthma from 1969 to 1999 were retrieved from the Swedish Hospital Discharge Registry. Psoriasis was positively associated with leukaemia, excluding chronic lymphocytic leukaemia, (odds ratio [OR]=1.6, 95% confidence interval [CI] 1.1-2.3) and non-Hodgkin's lymphoma (OR=1.6, 95% CI 1.3-2.1). Sjögren's syndrome increased the risks of all haematological malignancies combined (OR=4.0, 95% CI 2.3-7.0), and of non-Hodgkin's lymphoma (OR=6.4, 95% CI 3.5-12). These findings, together with increased risks of several haematological malignancies in autoimmune haemolytic anaemia and idiopathic thrombocytopenic purpura but not in asthma, suggest chronic autoimmunity and immune stimulation as mechanisms contributing to the development of haematological malignancies.