BACKGROUND: The natural history of venous thromboembolism (VTE) in patients with dementia has not been thoroughly studied. METHODS: We used the RIETE Registry data to assess the clinical characteristics, treatment strategies and outcome during the first 3 months after acute VTE in all immobilized patients with dementia. RESULTS: As of August 2011, 37988 patients had been enrolled, of whom 1316 (3.5%) had dementia. Most patients in both subgroups were initially treated with low-molecular-weight heparin (LMWH). Then, 48% of patients with dementia and 25% of those without dementia received LMWH as long-term therapy. During the first 3 months of anticoagulant therapy, patients with dementia had a higher incidence of fatal pulmonary embolism (PE): 4.0% vs. 1.2% (odds ratio: 3.3; 95% CI: 2.5-4.4) and fatal bleeding: 1.4% vs. 0.5% (odds ratio: 2.9; 95% CI: 1.8-4.6) than those without dementia. In demented patients initially presenting with PE, the incidence of fatal PE during the first week outweighed that of fatal bleeding (42 vs. 4 deaths), but from Day 8, the incidence of fatal PE was similar to the incidence of fatal bleeding. In patients initially presenting with deep vein thrombosis (DVT), there were 4 fatal PE and 8 fatal bleeding events. CONCLUSIONS: VTE patients with dementia had a high incidence of fatal PE and fatal bleeding. In those initially presenting with PE, the risk of dying of PE far outweighed that of fatal bleeding. In patients presenting with DVT alone, the risk of fatal PE was lower than that of fatal bleeding.