AimIn Japan, an additional system for patients with suspected or diagnosed dementia during hospitalization for physical illness began in 2016. We examined the clinical characteristics of older patients with suspected or diagnosed dementia during hospitalization.MethodsThe study participants consisted of 569 patients. Current age, sex, comorbidities, causative disease for admission, body mass index (BMI), blood tests, environment before admission, prescription history before admission and rate of delirium were examined. Simple regression analysis for high‐frequency diseases was carried out with the respective risk factors as independent variables. Multiple regression analysis was then carried out.ResultsInfection had the highest frequency in the causative diseases for admission. A total of 48% of patients had delirium during hospitalization. The delirium group had the highest frequency of being at home. In the infection group, BMI and serum albumin were lower in the non‐infection group (BMI 19.0 ± 3.7 vs 20.4 ± 4.1; P < 0.001, albumin 2.1 ± 0.7 vs 3.3 ± 0.6 g/dL; P < 0.001 respectively). White blood cell count and C‐reactive protein were higher than in the non‐infection group, respectively (white blood cell count 11181.4 ± 6533.3 vs 8765.8 ± 111 424.3/μL; P = 0.007, C‐reactive protein 8.6 ± 8.6 vs 3.0 ± 5.2 mg/dL; P < 0.001 respectively). Using independent factors associated with infection, multiple regression analysis was performed. BMI (P = 0.013), serum albumin (P < 0.001) and nursing home care before admission (P < 0.001) had significant correlations.ConclusionsAssessment of delirium and the environment before admission of older patients with suspected or diagnosed dementia when hospitalized are necessary. Furthermore, evaluation of nutrition might also contribute to reducing deterioration due to physical illness. Geriatr Gerontol Int 2024; ••: ••–••.