OBJECTIVES:  Study aims were to (1) determine the association of dementia with 30-day readmission, and (2) determine if this association varies by most common primary discharge diagnosis at the index hospitalization. METHODS:  This retrospective cohort study used data from the 2013 Nationwide Readmission Database. The cohort included 265,466 patients aged 65 and older. A 1:1 matching procedure by age, gender, discharge disposition, and emergency department use was used to match 132,733 patients with dementia to patients with no dementia. Descriptive statistics was used to determine top five primary diagnoses for patients with a 30-day readmission. RESULTS:  30-day readmission rates were higher among dementia patients (17.2%) compared to non-dementia patients (14.9%) (p< 0.01). In adjusted analysis, dementia was associated with 1.15 (95% CI=1.13-1.18) higher likelihood for 30-day readmission. The top five primary discharge diagnoses among patients with a 30-day readmission were septicemia, congestive heart failure, urinary tract infection, pneumonia, and hip fracture. Dementia was associated with higher odds for 30-day readmissions for patients with a primary discharge diagnosis of septicemia (OR=1.34, 95% CI=1.12-1.61) and pneumonia (OR=1.40, 95% CI=1.04-1.90), but not congestive heart failure (OR=1.29, 95% CI=0.96-1.75), urinary tract infection (OR=1.02, 95% CI=0.76-1.38) or hip fracture (OR=0.99, 95% CI=0.77-1.27). CONCLUSIONS: Dementia is associated with higher odds for 30-day readmissions regardless of their primary reason for admission. The risk of 30-day readmission is greater for patients with a primary diagnosis of septicemia or pneumonia at the index hospitalization. Care plans for specific conditions may reduce the risk for readmission associated with dementia.