Aim The present study verified the hypothesis that decreased skeletal muscle in older adults with Alzheimer's disease is related to Alzheimer's disease progression and decreased oral or swallowing function. Methods We investigated 232 patients with Alzheimer's disease (31 men, 201 women, average age 85.4 ± 5.9 years) in two regions in Japan. The patients provided basic information (sex and age), and were assessed for skeletal muscle index, dementia severity (clinical dementia rating), activities of daily living, nutritional status, oral status and swallowing function. Results Stratification by clinical dementia rating was as follows: clinical dementia rating 0.5:21 patients (9.0%), clinical dementia rating 1:85 patients (36.6%), clinical dementia rating 2:88 patients (37.9%) and clinical dementia rating 3:38 patients (16.3%). Alzheimer's disease severity was significantly related to skeletal muscle index. Logistic regression analysis showed that clinical dementia rating 3 (odds ratio 11.68, 95% confidence interval 4.52–30.20), body mass index < 18.5 (odds ratio 3.18, 95% confidence interval 1.27–8.00), calf circumference <30.5 cm (odds ratio 9.33, 95% confidence interval 2.01–43.27) and poor swallowing function (odds ratio 4.93, 95% confidence interval 1.10–22.04) had a significant effect on decreased skeletal muscle index. Conclusions Therefore, decreased skeletal muscle mass in patients with Alzheimer's disease requires strategies to manage swallowing dysfunction. Geriatr Gerontol Int 2016; 17: 402–409.