Myelodysplastic syndromes (MDS) and chronic myelomonocytic leukaemia (CMML) are associated with systemic inflammatory and autoimmune diseases (SIADs) in 10-30% of cases. The aims of this study were (i) to evaluate the prevalence of venous thromboembolism VTE in patients presenting with both MDS/CMML and SIADs, (ii) to describe risk factors associated with thrombosis, and (iii) to analyse the impact of VTE on overall survival and transformation to acute myeloid leukaemia in comparison to patients with MDS/CMML-associated SIADs without VTE.