Background and aims Many older adults with dementia are hospitalized for diagnoses other than dementia. We aimed to determine the prevalence of comorbid dementia among hospitalized older adults and evaluate its effects on their hospital course. Methods This retrospective case-control study reviewed the hospital records of all adults aged over 60 years admitted to one tertiary hospital in China from 2002 to 2012. In total, 34,888 patients meeting the age criterion were included. Patients admitted to departments of pediatrics, obstetrics and gynecology were not included. Demographic, clinical and outcome data from computerized discharge records were collected. Patients diagnosed with dementia at discharge by DSM-IV-TR criteria and MMSE scores formed the dementia group. All other patients were controls. Illness rating scale, comorbidities, mortality at discharge, dementia subtypes and long-term follow-up status for dementia patients were analyzed using comparative statistical methods (e.g., one-way ANOVA with Bonferroni pairwise comparison, Kruskal-Wallis and Mann-Whitney U test pairwise comparisons). Results A total of 918 patients (2.6 % prevalence) had comorbid dementia, including Alzheimer's disease (39.1 %) and vascular dementia (39.4 %). Neurologic and respiratory system diseases were the most common main diagnoses for patients with comorbid dementia, who also had a higher percentage of level III or IV severity of main illness compared to controls and longer hospital stays (both P < 0.01). Mortality at discharge included 9.80 % of the dementia group and 8.84 % of controls (P = 0.312). Conclusion Comorbid dementia has significant effects on hospital course of older adults with various main diagnoses, affecting length of stay, severity of illness, and mortality.