OBJECTIVE: To examine whether very late first-contact delusional disorder carries a risk for later development of dementia. METHODS: By linkage of the psychiatric and the somatic nationwide registers of all out- and in-patients with hospital contact in Denmark, we included all 60+ patients with first ever from 1 January 1994 to 31 December 2001 with the index main diagnosis: delusional disorder. First contact osteoarthritis patients as well as the general population were used as controls. A total of 1,437 patients with persistent delusional disorder and 7,302 patients with osteoarthritis were included. Median follow-up time until first diagnosis of dementia at discharge was 1.87 and 4.40 years, respectively. The probability of getting a dementia diagnosis was estimated using Poisson regression models with dementia as the outcome of interest. RESULTS: Patients with very late first-contact delusional disorder had an 8.14 (95% CI, 6.51; 10.19) times increased rate of subsequently developing dementia compared with very late first contact osteoarthritis patients. Compared with the general population the rate ratio was 5.49 (95% CI, 4.81; 6.26). CONCLUSION: Very late first-contact delusional disorder increases the risk of subsequently getting a diagnosis of dementia 5-8 times compared with osteoarthritis patients and the general population.